feat: deterministic simulation testing (DST) framework

[copyleftdev]

Summary

Adds a deterministic simulation testing framework to Effect, enabling reproducible concurrency testing with seeded scheduling.

Changes

packages/effect

• Scheduler.ts: add stepOne() to ControlledScheduler — executes exactly one pending task, returns boolean
• internal/fiberId.ts: replace hardcoded Date.now() in unsafeMake() with configurable clock source (setClockSource / resetClockSource)
• DSTScheduler.ts: public API for deterministic simulation — run(), runMany(), config types
• internal/dst/dstScheduler.ts: seeded PCG PRNG scheduler — same seed, same execution order
• internal/dst/dstRuntime.ts: assembles deterministic runtime (scheduler + clock + random)
• internal/dst/eventLog.ts: structured execution trace for replay
• internal/dst/livenessChecker.ts: post-hoc starvation, deadlock, and infinite loop detection

packages/vitest

• internal/dst.ts: it.dst() test primitive for multi-seed property-based concurrency testing

packages/dst

• DSTRunner: multi-seed runner with optional V8 profiling and failure shrinking
• V8Profiler: CPU profile capture with source map resolution
• GitBlame: commit attribution for hot code paths
• CascadeExporter: NDJSON export for external analysis tooling
• CommitClassifier: conventional commit classification and fix-ratio scoring
• ReportGenerator: HTML/JSON DST reports

Motivation

ControlledScheduler.step() drains all pending tasks in a single call — there is no way to execute one task and observe intermediate state. FiberId.unsafeMake() uses Date.now(), making fiber identity non-deterministic across runs.

Together, these prevent building FoundationDB/TigerBeetle-style deterministic simulation tests within Effect.

Prior work

• fix(effect): replace hardcoded Date.now() in FiberId.unsafeMake with configurable clock source #6167 (closed, v3 freeze) — FiberId configurable clock source
• feat(effect): add stepOne() to ControlledScheduler for fine-grained task execution #6168 (closed, v3 freeze) — stepOne() for ControlledScheduler

Both issues are confirmed still present on current main after the v4 merge.

Tests

• 9 proof-of-issue tests confirming both issues exist and the fixes resolve them
• 82 existing tests pass (Scheduler, FiberId, Fiber, FiberRef, concurrency)
• 7 DST-specific test suites (scheduler properties, fault injection, e2e, benchmark, vajra scoring)

Checklist

• Tests added
• Build passes (pnpm build on packages/effect)
• No regressions (82/82 existing tests pass)
• Follows Effect coding conventions

[changeset-bot]

🦋 Changeset detected

Latest commit: bde42b9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 37 packages

Name	Type
effect	Minor
@effect/vitest	Major
@effect/cli	Major
@effect/cluster	Major
@effect/dst	Major
@effect/experimental	Major
@effect/opentelemetry	Major
@effect/platform-browser	Major
@effect/platform-bun	Major
@effect/platform-node-shared	Major
@effect/platform-node	Major
@effect/platform	Major
@effect/printer-ansi	Major
@effect/printer	Major
@effect/rpc	Major
@effect/sql-clickhouse	Major
@effect/sql-d1	Major
@effect/sql-drizzle	Major
@effect/sql-kysely	Major
@effect/sql-libsql	Major
@effect/sql-mssql	Major
@effect/sql-mysql2	Major
@effect/sql-pg	Major
@effect/sql-sqlite-bun	Major
@effect/sql-sqlite-do	Major
@effect/sql-sqlite-node	Major
@effect/sql-sqlite-react-native	Major
@effect/sql-sqlite-wasm	Major
@effect/sql	Major
@effect/typeclass	Major
@effect/workflow	Major
@effect/ai	Major
@effect/ai-amazon-bedrock	Major
@effect/ai-anthropic	Major
@effect/ai-google	Major
@effect/ai-openai	Major
@effect/ai-openrouter	Major

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[tim-smart]

Effect v3 is under a feature freeze, feel free to move this contribution to the effect-smol repository.