A forge's size is written to the ledger and parent.json and listed by GET /api/v1/forges and MCP spawn #3094

Second half of #2995.

Done looks like:

• [ ] The size, small by default, is written to the ledger and to parent.json; a ledger row with no size reads as small.
• [ ] GET /api/v1/forges lists size for each forge, and the MCP spawn tool accepts size with the same validation.
• [ ] Tests that pin the exact key set of parent.json or the forges list gain size; the pull names them.
• [ ] src/web/ui.ts and src/web/pages/new-forge.ts are not changed.

Files: src/spawn/door.ts, src/spawn/family.ts or the ledger module, src/http/mcp.ts, src/http/api.ts, test/spawn-sizes.test.ts.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3117 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: @scout's run ended without a pull: the pull closed

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #3117 closed)
2. @scout's run ended without a pull: the pull closed

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #4381 (the size is written to the ledger and parent.json; tests that pin the key set are updated, which is the likely reason the check stayed red) and #4382 (the API list and MCP spawn). #3095 and #2996 now wait on #4382.