fix(sqlite): isolate authenticated local commits

[marcus-pousette]

Problem

SQLite must not hold a savepoint while asynchronous authorization runs, and a committed authenticated operation must never be separated from its exact proof.

What changes

Authenticated local writes:

1. prepare an exact read-only proposal
2. call authorizeLocalOps, which returns one complete proof
3. validate and snapshot the proposal, 64-byte signature, and 16-byte token reference
4. atomically revalidate and commit operation, materialization, and proof
5. prepare and reauthorize only after an explicit optimistic conflict, up to four attempts

Policy, codec, proof, database, and listener failures are terminal. Events are emitted only after commit.

Clean-slate choice

There is no optional proof hook, missing-reference branch, proofless authenticated commit, or old savepoint-across-auth path.

Fast paths

• Writes without authSession retain the single immediate SQL call.
• No SQLite transaction or lock is held during authorization.
• Reconcile reuses the exact retained proof and cannot mint one later.

Verification

• sqlite-node: 38/38 under Node 20
• native extension release build
• interface, benchmark, conformance, and sqlite-node TypeScript builds
• Rustfmt, Prettier, and diff checks

Stack

Stacked on #194 and completes the SQLite authenticated local-write path.