feat(chat): render an answer's state, and keep the part that arrived

[mrsibe]

Closes #142. Fourth step of the chat execution epic (#138): #139 decided what a turn is, #140 gave it an owner, #141 put the SDK's own events on the wire — and the transcript was still rendering isStreaming booleans and a finishReason copied into the metadata bag, because it had nothing else to read.

It has the record now.

What changes

• shared/utils/answerState.ts is the only mapping from status to what the reader sees: live or ended, which line explains it, whether partial text is kept. One function, so the answer that is arriving and the same answer after a restart cannot drift — both hand the same fields to the same code. That is the acceptance item "running-turn overlay and reloaded-from-database render the same thing" expressed as a property rather than a promise.
• Four states get one muted line beside the answer (text-xs text-subtle-foreground, the treatment the retrieval lines already use):

status	the line
truncated	the output ceiling ended it — a completion with a known defect
blocked	the provider refused it; not the model's own decision
aborted	the reader stopped it
failed	generation broke, with the reason

None is an error card, and none replaces the text. completed says nothing; a row written before the column existed says nothing either, because unknown is not a claim.

• A failed or stopped turn keeps its partial answer. It used to be replaced by ❌ Error: …, which threw away the only thing the reader had. This is the visible change of the PR.
• isStreaming / isReasoningStreaming are gone, from the message type and from the store. Liveness comes from status; "the model is still thinking" comes from the assembled message's own part state (isReasoningLive), so the store no longer tracks it from events. The renderer's placeholder starts as pending and becomes streaming on the first event — the same transition the execution makes in Main.
• The fix(chat): report a broken stream as a failure, not a finished answer #137 bridge is removed. The provider's finish reason is no longer copied into metadata: the record has a finish_reason column, and one fact belongs in one place. parseFinishReason goes with it, and the manager test now asserts the metadata bag stays free of it.
• loadMessages no longer patches an in-flight message with an isStreaming flag — the row's own status is already streaming while the turn runs, so the overlay supplies only the text that is not persisted yet.

A race the tests caught

The outcome event can arrive while the last chunks are still being assembled. Applying it immediately dropped exactly the partial answer this PR exists to keep, so a stopped turn could render empty. The assembler now exposes drained, and the outcome is applied after it resolves. Found by test/chatTurnEvents.test.ts, which is the reason that test exists.

Verification

npm test	324 pass, 0 fail — 5 new in test/answerState.test.ts
npm run typecheck	clean
npm run check:design	no violations
npm run build	bundles (renderer unchanged at 6,093.40 kB)
npm run build:unpack + smoke:packaged	PASS — 23 checks (main changed)
npx eslint <changed files>	0 problems

DESIGN.md gains the answer-state contract next to the per-zone state table, and docs/architecture.md's chat-execution section already describes the protocol this renders from.

Not hand-verified: the lines themselves in the GUI (both themes). They are three i18n strings per locale behind one pure mapping; the states they render are the ones the tests drive.

What is left of the epic

#143 (stream reliability) is the last child. With this merged, every Definition-of-Done item in #138 apart from that issue is satisfied.