Commit every message on one serial queue, under a key

[andreisavu]

PR 2 of the stack that SPLIT_PLAN.md on #48 cuts from that branch. Branches from main after #49.

The one idea

The record becomes a log, and every message commits on one serial queue.

• log/log.ts replaces record.ts. RoomLog.commit runs the key check, the readThrough check, the append and the cache update inside one link of a promise chain, and what the room does with a fresh message runs there too, before the next commit starts. A message exists when its write is confirmed: a write that fails leaves nothing on the record, nothing on the stream, and wakes nobody. A repeated key hands back the message the first commit landed. Rule 5 is enforced where the write happens.
• Keys. A seat's say and the assistant's two tools take Pi's tool call id as the key; deliver({ key }) takes the key the host passes, or a fresh one. Message gains key.
• The composition is checked against the replayed record, so a name the record knows as a person is refused at the first call, and a visit is checked after the replay.
• The entries on the storage do not change: messages only. Logs written before this PR resume unchanged.

The tests that pin it

• test/log.test.ts: a repeated key lands once and returns the first message; of two commits under one readThrough, one lands and the other is refused with what it missed and consumes no seq; a message is invisible until its write resolves; a commit whose write fails leaves nothing, and the next one takes its seq.
• test/session.test.ts: the new cases on deliver with a key and on a failed write.
• test/presence.test.ts: reworked onto the queue, same claims.
• The invariants now check that every key names one message.

Extract

git cherry-pick -x 0f44d6c on #49, with the path change to log/log.ts. Two conflicts, both against #49's layout: the import block in session.ts and the layering sentence in agent.md. Two additions the plan did not list: the simple faultyOpener returns to test/support/storage.ts because the fourth log test needs it (#49 had dropped it; PR 6 replaces it with the tapped version), and the layer table in docs/toolchain.md §1 gains the log/ row.

What a later PR adds on top

• PR 4: the close and composition entry kinds, Row, RowData and write().
• PR 5: the lease entry kind.
• PR 6: the doubt path, cursor, known, found, the "in doubt" tests.
• PR 12: the checkpoint entry kind and compact.

Checks

pnpm format && pnpm check is green, 120 tests. Every doc link resolves. The build's three TS4023 lines from test/live/support.ts are on main too.

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