feat(effect): add stepOne() to ControlledScheduler for fine-grained task execution

[copyleftdev]

Problem

ControlledScheduler.step() drains all pending tasks across all priority buckets in a single call:

step() {
  const tasks = this.tasks.buckets
  this.tasks.buckets = []          // takes everything
  for (const [_, toRun] of tasks)  // runs everything
    for (let i = 0; i < toRun.length; i++)
      toRun[i]()
}

There is no way to execute exactly one task and observe the intermediate state. This makes it impossible to reproduce concurrency bugs in a controlled environment — you can't step through individual task interleavings to find the specific ordering that triggers the issue.

Relevant bugs that are hard to reproduce without fine-grained stepping:

• fix semaphore race condition where permits could be leaked #6081 — semaphore race condition where permits could be leaked
• @effect/workflow: Effect.all with concurrency > 1 wrapping Activity.make causes deadlock during replay #6014 — workflow deadlock during durable replay with concurrency > 1
• fix: resolve batched request resolver defects hanging consumer fibers #6139 — batched request resolver defects hanging consumer fibers

Fix

Add stepOne() to ControlledScheduler:

stepOne(): boolean {
  const buckets = this.tasks.buckets
  for (let i = 0; i < buckets.length; i++) {
    const [_, tasks] = buckets[i]!
    if (tasks.length > 0) {
      const task = tasks.shift()!
      if (tasks.length === 0) buckets.splice(i, 1)
      task()
      return true
    }
  }
  return false
}

• Executes exactly one task from the highest-priority bucket
• Returns true if a task was executed, false if empty
• step() behavior unchanged
• 1 file, 15 lines added, purely additive

Test plan

• Schedule.test.ts — 82 tests pass
• Effect/scheduling.test.ts — 2 tests pass
• Fiber.test.ts — 16 tests pass
• Zero regressions

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 2854878

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

This PR includes changesets to release 36 packages

Name	Type
effect	Minor
@effect/cli	Major
@effect/cluster	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/vitest	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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