feat(materialization): expose committed local signer

[marcus-pousette]

Goal

Expose the verified signer public key on materialization changes produced by authenticated local SQLite writes.

Applications can attribute the committed change without re-reading the proof store or trusting metadata supplied separately by an auth session.

What changes

• After the authenticated optimistic commit succeeds, derive the signer from committedOp.meta.id.replica.
• Annotate only outcome changes whose source operation exactly matches that committed operation.
• Snapshot the public-key bytes before emitting the event.
• Leave unauthenticated writes and replay-derived changes unannotated.
• On an optimistic conflict, expose only the fresh operation that actually commits.

Clean-slate choice

There is no optional authSession.signer, signer decoder, savepoint bridge, alternate commit path, or compatibility fallback. The signer identity comes from the exact operation whose proof and operation committed atomically in #195.

Remote signer propagation remains separate because it must come from the proof accepted at the remote commit boundary.

Fast paths

• Unauthenticated writes perform no signer lookup or annotation.
• Authenticated writes reuse the already-returned committed operation; there is no database read, proof-store query, transaction, or extra signature verification.
• Only the small committed materialization outcome is mapped.

Size

4 files, +100/-18 relative to #195. Production behavior is 34 changed lines, roughly half the stale draft's signer bridge; the remaining diff is focused coverage.

Verification

• relevant TypeScript builds
• sqlite-node conformance under Node 20: 34/34
• focused coverage for real capability-backed auth, unauthenticated writes, denial/no event, and optimistic conflict retry
• Prettier and diff checks

Stack

Rebuilt cleanly on #195.

[marcus-pousette]

Moving this back to draft after the repository-wide audit. The current branch still holds a SQLite savepoint across async authorization, which the #193-#195 stack intentionally replaces, and it does not wire signer metadata from a real auth session. Rebuild this after #195 so signer metadata is attached only to the revalidated committed outcome, validate the real 32-byte signer key, and cover the actual session integration.