refactor(core): use Latch for MCP startup gate

[kitlangton]

What

Each MCP server entry's startup signal is a pure open-once gate: it is only ever completed with Exit.void — deliberately, even on registration failure or replacement interruption, so readers cannot hang ("Settle startup even when registration fails..." comment). A Deferred<void> leaves "never delivers a failure" as a convention enforced by three completion sites; Latch makes it structural — the type cannot fail or interrupt waiters.

How

packages/core/src/mcp/index.ts:

• ServerEntry.startup is now Latch.Latch, created with Latch.makeUnsafe() (starts closed).
• The three completion sites collapse to entry.startup.open / entry.startup.openUnsafe(); the ensuring(Effect.sync(() => Deferred.doneUnsafe(..., Exit.void))) wrapper becomes plain ensuring(entry.startup.open).
• The four waiters use entry.startup.await.

No behavior change. Found by a repo-wide audit of Deferred<void>-as-gate patterns; sibling PRs convert the other pure-gate sites.

Testing

• packages/core: bun typecheck, bun test test/mcp.test.ts test/mcp-instructions.test.ts test/mcp-oauth.test.ts test/mcp-import-boundary.test.ts (38 pass), and the full bun run test suite (0 fail across 1908 tests).