feat(rpc): expose native shared state for custom channels

[dvcolomban]

Background (Why)

Custom RPC channels can already use createRpcClient() and createRpcServer(), but the native shared-state implementations are not public. Channel bindings otherwise have to copy that synchronization logic or construct a full node context.

Changes (What)

Export the existing implementations through devframe/rpc/shared-state, accepting the native RPC members they use. Subscription handlers read the calling connection's birpc metadata, so custom channels and the existing node context share the same subscription behavior. A failed initial snapshot now rejects instead of leaving its promise pending.

Build the RPC entry points with the browser-safe graph and include them in the browser-import guard. Existing implementation files stay in place. The channel binding still owns peer authentication, serialization and disconnection.

This draft is now limited to RPC and shared state. JSON rendering is in #411 and baseline diagnostic snapshot repairs are in #412.

Verification (Testing)

Six focused tests pass, including two real MessageChannel peers through both custom channels and the native node context. They cover subscription isolation, writes, disconnection and rejected initial snapshots. The devframe build, package type check, scoped lint and Knip pass. The browser-import guard finds no forbidden imports across 31 reachable chunks and 20 entry points.

The new and changed API snapshots were generated from fresh artifacts. The unchanged baseline has three stale diagnostic snapshots, confirmed in base CI; their repair is separate from this feature. Full-repository validation runs in CI.

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