Vendor + bundle @comapeo/core types for core-react-native

Context

The RPC surface exposed by this module mirrors @comapeo/core's MapeoManager API, but the TypeScript types consumers see are hand-maintained / not derived from core. We want the published types to track core so the frontend gets accurate types over the RPC boundary without core-react-native taking a hard runtime dependency on core's source layout.

Scope

• Decide the mechanism: vendoring core's .d.ts, generating a trimmed type bundle at build time, or re-exporting from a pinned @comapeo/core version.
• Wire it into the build (scripts/build-backend.ts / package exports) so build/index.d.ts reflects the bundled types.
• Pin the core version the types are derived from and document how to bump it.
• Ensure attw / publint stay green (see #88).

Notes

Milestone-doc item: "Vendoring + bundling of types from core for core-react-native". Relates to #1 (Date objects over RPC) since that's a place where our types intentionally diverge from core.

Start with scripts/build-backend.ts, the package exports, and the generated build/index.d.ts to understand how published types are assembled. Compare the proposed vendoring, build-time bundling, and pinned re-export mechanisms, then verify that the chosen approach documents core version bumps and keeps attw and publint green.