[codex] Lazily create route transform worker slots

[ScriptedAlchemy]

Summary

• Lazily allocate route transform worker slots instead of creating the full pool the first time the executor is used.
• Keep task scheduling stable while only creating worker instances for slots that receive scheduled work.
• Add focused test coverage for lazy worker-slot creation.

Why

The original worker executor created every route transform worker at the first parallel transform touchpoint. Lazy slot allocation reduces the dev startup cliff and gives the follow-up prewarm PR a cheaper baseline to build from.

Validation

Validated before stacking this PR:

• pnpm exec tsc --noEmit
• pnpm test tests/parallel-route-transforms.test.ts tests/index.test.ts tests/features.test.ts tests/build-output-transforms.test.ts
• pnpm build

[github-actions]

Benchmark Results

Compared PR head a3781b2 against base 7fe442c.

Total median wall time: 9.77s -> 9.72s (-0.5%, 1.01x speedup)
Compiler ready median: 9.77s -> 9.72s (-0.5%)
Route load median: 1.96s -> 1.99s (+1.5%)

Benchmark	Base total	Head total	Delta	Head ready	Head routes	Speedup	Head RSS p95
large-355-ssr-esm	9.77s	9.72s	-0.5%	9.72s	1.99s	1.01x	-

Profile: large; mode: dev; iterations: 10; warmup: 0.
Workflow run