fix(ui): stop remounting a turn's answer as it settles

[Astro-Han]

Fixes the only red e2e on main: quote-selection.spec.ts never sees .maka-quote-actions.

The problem

The assistant answer's React key was derived from its first timeline entry. While a turn runs that entry is the Processing fold; when the last tools group is projected away the fold dissolves, the first entry changes, and so does the key — React unmounts the whole answer and mounts an identical-looking copy, taking with it the scroll position, any open disclosure, and any text Selection held inside.

timeline-fold.ts already made this argument for the Processing block itself — its id comes from the preceding boundary precisely so it "survives the first tool being projected away without remounting". The answer element was still keyed by a guess at its first child.

Changes

1. fix(ui): stop remounting a turn's answer as it settles
Key the assistant message by what it replies to — the steering message that opened it, or a disjoint assistant-opening for the turn's first answer (a steering id is any string, so a bare sentinel could collide with a real one). Fold the streaming and settled bubbles into one component so settling changes attributes rather than swapping component types, which is itself an unmount. Adds chat-turn-answer-identity.test.tsx, which renders TurnView into a real DOM and asserts the answer element isSameNode across the transition.

2. refactor(ui): narrow MessageBody to the user message it now renders
After the merge every caller passed role="user", so the generic prop and the assistant branch were unreachable. Most of that diff is de-indentation.

3. fix(ui): announce settlement from the answer's phase
The merged component spelled the answer's three lives — historical, streaming, settled — as two independent booleans, which made live: false, streaming: true representable and pushed gating of every live-only prop out to the call site. One was already forgotten (settledText reached historical bubbles). The same shape produced a real defect: settlement was inferred from a settledRef reset by [text, streaming], so a bubble mounted from history — already streaming: false — consumed the one-shot ref; when that same message id was promoted to a completed live projection neither dependency had changed, the announcement never fired, and the answer wore the live marker until the shell's 1000ms fallback. Collapsed to one phase field: historical bubbles cannot carry live-only props at all, and settlement is the edge into settled.

4. docs(ui): say what the remount fix fixes, and record what it does not
The comments claimed a user-visible outcome the code does not produce. See below.

The gap this does not close

A Selection taken inside a still-streaming answer dies anyway: the markdown renderer rebuilds the paragraph's inline fragments when the stream closes, and the browser discards the Selection those nodes held — without a selectionchange, so the quote hook's read 350ms after pointer release finds nothing. Pinning the isStreaming and settledText props that drive the rebuild still reproduces it; it happens inside @astryxdesign/core.

The e2e therefore selects from a settled answer — it exists to pin the pointer-capture contract, not Selection survival across a stream close, and no capture/release assertion was dropped. The lost precondition is recorded as a test.fixme beside it rather than only in this description. Closing it needs an upstream fix, or a decision to snapshot the quote at pointerup — which would show a quote bar over text whose highlight the browser has already erased. That is a product call.

Verification

• quote-selection passes 5/5 consecutive runs (5/5 failures before this change, same invocation)
• Desktop e2e: 16 passed, 1 skipped (the fixme above)
• @maka/ui 133/133, typecheck and format:check clean
• Each new test was checked for teeth by reverting the behaviour it pins and confirming it goes red
• Unrelated: slash-command-menu.spec.ts:87 is flaky on origin/main too — 6 failures in 20 runs there, on an untouched code path

Generated-by: Claude Code