feat(speculation): add enumerator + dependency-limit extensions

Add the first speculation seam and its limit counterpart from the speculation RFC, as vendor-agnostic extension interfaces under submitqueue/extension/speculation/.

enumerator: given a batch and its ordered active dependencies, mechanically lists the candidate Base/Head paths — pure, deterministic, and purely structural. It sets no score and no status; the controller stamps status on persist and the scorer fills score. (Its earlier scoring responsibility is dropped here, per the RFC, and moves to the scorer seam.)

dependencylimit: the "how much" policy bounding how many active (in-flight) dependencies a batch may speculate over. It is the eligibility gate before enumeration; unlike the other speculation limits it is controller-held rather than injected into a seam, keeping the enumerator pure. The value is signal-driven, not a fixed constant.

Each follows the repo extension contract (conflict.Analyzer reference shape): Factory.For(Config) (T, error) with Config carrying only QueueName; behavioral knobs and limit signals are integrator-injected at construction. Includes READMEs, gomock packages, and Makefile mock-gen wiring. Interfaces only; concrete impls and controller wiring are deferred.

Author: behinddwalls

Makefile

@@ -364,7 +364,7 @@ local-stovepipe-stop: ## Stop the Stovepipe service
 
 mocks: ## Generate mock files using mockgen
 	@echo "Generating mocks..."
-	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./platform/extension/counter/... ./platform/extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/pusher/... ./submitqueue/extension/scorer/... ./submitqueue/extension/conflict/... ./submitqueue/extension/speculation/enumerator/... ./submitqueue/extension/speculation/dependencylimit/... ./submitqueue/extension/validator/... ./platform/consumer/... ./submitqueue/core/changeset/... ./stovepipe/extension/storage/... ./stovepipe/extension/sourcecontrol/...
+	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./platform/extension/counter/... ./platform/extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/pusher/... ./submitqueue/extension/scorer/... ./submitqueue/extension/conflict/... ./submitqueue/extension/speculation/enumerator/... ./submitqueue/extension/speculation/dependencylimit/... ./submitqueue/extension/validator/... ./platform/consumer/... ./stovepipe/extension/storage/... ./stovepipe/extension/sourcecontrol/...
 	@echo "Mocks generated successfully!"
 
 proto: ## Generate protobuf files from .proto definitions