A spawn plan accepts an optional size (small, medium, large) and old plan hashes stay byte-identical #3093

First half of #2995 (parked: its check failed on the shared /map timing flake, not its code; this half is smaller still).

Done looks like:

• [ ] src/spawn/sizes.ts holds one table mapping small | medium | large to CPU, memory MiB and disk MiB, and parseSize(v) returns a size or a one-sentence error.
• [ ] size is optional in the spawn input; src/spawn/plan.ts leaves it out of the plan when not given, and a test pins one old plan hash unchanged.
• [ ] An unknown size is a 422 checked after the existing field checks.
• [ ] src/web/ui.ts and src/web/pages/new-forge.ts are not changed.

Files: src/spawn/sizes.ts, src/spawn/plan.ts, src/actions/spawn.ts, test/spawn-sizes.test.ts.