Expose local auth signer in materialization events

[marcus-pousette]

Summary

• add a MaterializationSigner type and optional authSession.signer hook for local writes
• annotate SQLite local-write materialization changes with source.signer.publicKey after auth succeeds
• cover sqlite-node local auth materialization events with signer metadata

Notes

• This is local-write plumbing only. Remote sync signer metadata still needs verified signer info to cross the sync verifyOps / applyOps boundary.
• Wiring createTreecrdtAuthSession to expose the signer should wait for Simplify auth session options #141 so we avoid churn against the session options cleanup.
• Remote signer propagation should wait for the sync API work in Add outbound sync API #137/Add inbound sync API #138 to settle.