[Tech debt]: [P3] serialization.ts and Simulation.ts sit exactly at the 500-line guard ceiling

> Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story engine-file-size-margin).
> Priority P3 (do-first order) | Severity: low | Status: idea | Epic: code health | Type: design-decision | Recorded: 2026-07-21

Problem / motivation

Acceptance Auditor margin note from the scenery review (v1.72.0). The founding-seed save field (RNG.initialSeed) landed with src/engine/sim/serialization.ts and src/engine/Simulation.ts at exactly 500 lines, the file-size guard's ceiling. The next one-line addition to either forces a split mid-PR, the way this PR had to move SerializedGame out of types.ts into src/engine/serializedGame.ts (type-only re-export keeps the "./types" doorway). Next natural seams: serialization's serializeUnit/serializeTransport pair or the coerce helpers could move beside sim/coerce.ts; Simulation's thin delegation blocks could shed a group. No behavior change involved; do it as a pure move under the golden-master net when either file next needs room.

Start with src/engine/sim/serialization.ts and src/engine/Simulation.ts, then read the golden-master tests and the existing src/engine/serializedGame.ts move. Compare the suggested seams around serializeUnit/serializeTransport, the coerce helpers, or Simulation's delegation blocks. Done means a pure move that preserves behavior and leaves both guarded files below the 500-line ceiling.