feat(platform): add pipeline.Construct engine

## Summary

- Introduces `platform/pipeline` with the typed `Construct[D]` engine
- `Stage[D]` struct declares pipeline topology as a typed table (key, name, consumer group, controller constructor, optional DLQ)
- Engine builds topic registry, creates primary + DLQ consumers, eagerly constructs all controllers, pairs DLQ stages automatically
- Returns a `lifecycle.Component` for ordered start/stop
- Options: `TopicNames`, `Classifiers`, `PublishOnly`, `ExtraComponents`
- Pure addition — no existing code changes

Step 2 of the [Modular Queue Wiring RFC](https://github.com/uber/submitqueue/blob/main/doc/rfc/submitqueue/modular-queue-wiring.md). Stacked on #402.

## Test plan

- [x] 10 unit tests: single stage, DLQ pairing, multiple stages, empty stages error, controller creation failure, DLQ creation failure, publish-only topics, topic name overrides, resolveTopicName table test, dlqTopicKey, buildTopicConfigs
- [x] `bazel test //platform/pipeline:go_default_test` passes
- [x] `make gazelle` — BUILD.bazel in sync
- [x] `make fmt` — code formatted

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JamyDev

platform/pipeline/BUILD.bazel

@@ -0,0 +1,32 @@
+load("@rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["pipeline.go"],
+    importpath = "github.com/uber/submitqueue/platform/pipeline",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//platform/consumer:go_default_library",
+        "//platform/errs:go_default_library",
+        "//platform/extension/messagequeue:go_default_library",
+        "//platform/lifecycle:go_default_library",
+        "@com_github_uber_go_tally//:go_default_library",
+        "@org_uber_go_zap//:go_default_library",
+    ],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = ["pipeline_test.go"],
+    embed = [":go_default_library"],
+    deps = [
+        "//platform/consumer:go_default_library",
+        "//platform/extension/messagequeue:go_default_library",
+        "//platform/extension/messagequeue/mock:go_default_library",
+        "@com_github_stretchr_testify//assert:go_default_library",
+        "@com_github_stretchr_testify//require:go_default_library",
+        "@com_github_uber_go_tally//:go_default_library",
+        "@org_uber_go_mock//gomock:go_default_library",
+        "@org_uber_go_zap//:go_default_library",
+    ],
+)

platform/pipeline/pipeline.go

@@ -0,0 +1,281 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package pipeline provides a typed engine for assembling queue-driven
+// service pipelines from declarative data. A service declares its topology
+// as a []Stage[D] table and its dependencies as a Deps struct; Construct
+// builds all consumers, registers controllers, pairs DLQ stages, and
+// returns a single lifecycle.Component the host drives with Start/Stop.
+package pipeline
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/uber-go/tally"
+	"github.com/uber/submitqueue/platform/consumer"
+	"github.com/uber/submitqueue/platform/errs"
+	extqueue "github.com/uber/submitqueue/platform/extension/messagequeue"
+	"github.com/uber/submitqueue/platform/lifecycle"
+	"go.uber.org/zap"
+)
+
+// timeNow is a hook for tests to control time. Production uses time.Now.
+var timeNow = time.Now
+
+// Stage is one row of a service's topology table. D is the service's Deps type.
+type Stage[D any] struct {
+	// Key is the stage's logical topic key (e.g. topickey.TopicKeyStart).
+	// The engine maps it to a physical topic name via the TopicNames option.
+	Key consumer.TopicKey
+
+	// Name is the physical topic name for this stage (e.g. "start").
+	// Used as the default when no TopicNames override is provided.
+	Name string
+
+	// ConsumerGroup is the consumer group suffix for this stage's subscription
+	// (e.g. "orchestrator-start").
+	ConsumerGroup string
+
+	// New builds the stage's controller from the service's Deps. The engine
+	// calls it once, eagerly, inside Construct — so a nil/missing dependency
+	// fails at boot with the stage's name on it, never mid-delivery.
+	New func(D) (consumer.Controller, error)
+
+	// DLQ, when non-nil, declares "this stage dead-letters". The engine then
+	// derives the paired DLQ topic (<topic>_dlq, retry budget, DLQ-of-DLQ
+	// disabled) AND registers this reconciler on the DLQ consumer. Declaring
+	// one without getting the other is impossible — that's the invariant.
+	DLQ func(D) (consumer.Controller, error)
+}
+
+// PublishOnlyTopic declares a topic the service publishes to but does not
+// consume. The engine registers it in the TopicRegistry so controllers
+// can publish to it, but creates no subscription or controller.
+type PublishOnlyTopic struct {
+	// Key is the logical topic key.
+	Key consumer.TopicKey
+
+	// Name is the physical topic name.
+	Name string
+}
+
+// options holds the resolved configuration for a Construct call.
+type options struct {
+	topicNames      map[consumer.TopicKey]string
+	classifiers     []errs.Classifier
+	publishOnly     []PublishOnlyTopic
+	extraComponents []lifecycle.Component
+}
+
+// Option configures a Construct call.
+type Option func(*options)
+
+// TopicNames provides a mapping from logical topic keys to physical topic
+// names. Keys not present in the map fall back to the Stage.Name default.
+func TopicNames(m map[consumer.TopicKey]string) Option {
+	return func(o *options) { o.topicNames = m }
+}
+
+// Classifiers sets the error classifiers for the primary consumer's
+// ErrorProcessor. DLQ consumers always use AlwaysRetryableProcessor.
+func Classifiers(c ...errs.Classifier) Option {
+	return func(o *options) { o.classifiers = c }
+}
+
+// PublishOnly adds topics the service publishes to but does not consume.
+func PublishOnly(topics ...PublishOnlyTopic) Option {
+	return func(o *options) { o.publishOnly = append(o.publishOnly, topics...) }
+}
+
+// ExtraComponents adds lifecycle components that are started before
+// consumers and stopped after them.
+func ExtraComponents(c ...lifecycle.Component) Option {
+	return func(o *options) { o.extraComponents = append(o.extraComponents, c...) }
+}
+
+// dlqTopicKey returns the DLQ topic key for a primary stage key.
+// Matches the convention in submitqueue/orchestrator/controller/dlq.TopicKey.
+const dlqTopicSuffix = "_dlq"
+
+func dlqTopicKey(primary consumer.TopicKey) consumer.TopicKey {
+	return consumer.TopicKey(string(primary) + dlqTopicSuffix)
+}
+
+// Construct is the single assembly function for a queue-driven service.
+// It builds the topic registry, creates primary and DLQ consumers,
+// eagerly constructs all controllers, and returns a lifecycle.Component
+// that starts and stops everything in the correct order.
+//
+// The returned Component starts in this order:
+//  1. Extra components (infrastructure)
+//  2. Primary consumer (work-accepting)
+//  3. DLQ consumer (reconciliation)
+//
+// Stop reverses the order: DLQ consumer drains first, then primary, then
+// infrastructure.
+func Construct[D any](
+	logger *zap.SugaredLogger,
+	scope tally.Scope,
+	queue extqueue.Queue,
+	subscriberName string,
+	deps D,
+	stages []Stage[D],
+	opts ...Option,
+) (lifecycle.Component, error) {
+	if len(stages) == 0 {
+		return nil, fmt.Errorf("pipeline: at least one stage is required")
+	}
+
+	o := &options{}
+	for _, opt := range opts {
+		opt(o)
+	}
+
+	// Build topic configs for the registry.
+	configs, err := buildTopicConfigs(queue, subscriberName, stages, o)
+	if err != nil {
+		return nil, err
+	}
+
+	registry, err := consumer.NewTopicRegistry(configs)
+	if err != nil {
+		return nil, fmt.Errorf("pipeline: failed to create topic registry: %w", err)
+	}
+
+	// Create the primary consumer with user-provided classifiers.
+	primaryProcessor := errs.NewClassifierProcessor(o.classifiers...)
+	primary := consumer.New(logger, scope, registry, primaryProcessor)
+
+	// Create the DLQ consumer with always-retryable processor.
+	dlq := consumer.New(logger, scope, registry, errs.AlwaysRetryableProcessor)
+
+	hasDLQ := false
+
+	// Eagerly construct and register all controllers.
+	for _, s := range stages {
+		ctl, err := s.New(deps)
+		if err != nil {
+			return nil, fmt.Errorf("pipeline: stage %s: failed to create controller: %w", s.Key, err)
+		}
+		if err := primary.Register(ctl); err != nil {
+			return nil, fmt.Errorf("pipeline: stage %s: failed to register controller: %w", s.Key, err)
+		}
+
+		if s.DLQ != nil {
+			rec, err := s.DLQ(deps)
+			if err != nil {
+				return nil, fmt.Errorf("pipeline: stage %s dlq: failed to create controller: %w", s.Key, err)
+			}
+			if err := dlq.Register(rec); err != nil {
+				return nil, fmt.Errorf("pipeline: stage %s dlq: failed to register controller: %w", s.Key, err)
+			}
+			hasDLQ = true
+		}
+	}
+
+	// Compose the lifecycle group.
+	members := make([]lifecycle.Component, 0, len(o.extraComponents)+2)
+	members = append(members, o.extraComponents...)
+	members = append(members, &consumerComponent{name: "primary", c: primary})
+	if hasDLQ {
+		members = append(members, &consumerComponent{name: "dlq", c: dlq})
+	}
+
+	return lifecycle.NewGroup(members...), nil
+}
+
+// buildTopicConfigs constructs the []consumer.TopicConfig from stages and options.
+func buildTopicConfigs[D any](
+	queue extqueue.Queue,
+	subscriberName string,
+	stages []Stage[D],
+	o *options,
+) ([]consumer.TopicConfig, error) {
+	// Pre-size: each stage gets a primary config + optional DLQ config,
+	// plus publish-only topics.
+	configs := make([]consumer.TopicConfig, 0, 2*len(stages)+len(o.publishOnly))
+
+	for _, s := range stages {
+		topicName := resolveTopicName(s.Key, s.Name, o.topicNames)
+
+		configs = append(configs, consumer.TopicConfig{
+			Key:   s.Key,
+			Name:  topicName,
+			Queue: queue,
+			Subscription: extqueue.DefaultSubscriptionConfig(
+				subscriberName, s.ConsumerGroup,
+			),
+		})
+
+		if s.DLQ != nil {
+			configs = append(configs, consumer.TopicConfig{
+				Key:   dlqTopicKey(s.Key),
+				Name:  topicName + dlqTopicSuffix,
+				Queue: queue,
+				Subscription: extqueue.DLQSubscriptionConfig(
+					subscriberName, s.ConsumerGroup+"-dlq",
+				),
+			})
+		}
+	}
+
+	for _, p := range o.publishOnly {
+		topicName := resolveTopicName(p.Key, p.Name, o.topicNames)
+		configs = append(configs, consumer.TopicConfig{
+			Key:   p.Key,
+			Name:  topicName,
+			Queue: queue,
+		})
+	}
+
+	return configs, nil
+}
+
+// resolveTopicName returns the override name if present, otherwise the default.
+func resolveTopicName(key consumer.TopicKey, defaultName string, overrides map[consumer.TopicKey]string) string {
+	if overrides != nil {
+		if name, ok := overrides[key]; ok {
+			return name
+		}
+	}
+	return defaultName
+}
+
+// consumerComponent adapts consumer.Consumer to lifecycle.Component.
+// Consumer.Stop takes a timeoutMs int64; Component.Stop takes a context.
+// We derive the timeout from the context's deadline if set, defaulting to 30s.
+type consumerComponent struct {
+	name string
+	c    consumer.Consumer
+}
+
+func (a *consumerComponent) Start(ctx context.Context) error {
+	return a.c.Start(ctx)
+}
+
+func (a *consumerComponent) Stop(ctx context.Context) error {
+	const defaultStopTimeoutMs = 30000
+	timeoutMs := int64(defaultStopTimeoutMs)
+	if deadline, ok := ctx.Deadline(); ok {
+		remaining := deadline.Sub(timeNow())
+		if remaining > 0 {
+			timeoutMs = remaining.Milliseconds()
+		} else {
+			timeoutMs = 0
+		}
+	}
+	return a.c.Stop(timeoutMs)
+}