refactor(ui): unify streaming (live) and committed answer onto one render path

[Astro-Han]

Summary

Unify the streaming (live) and settled (committed) chat answer onto a single render path, so the streaming→settled handoff is structurally seamless instead of held pixel-neutral by hand. Today one logical assistant answer is rendered by two different DOM subtrees; the follow-up materializes the active streaming answer into a transient turn that reuses TurnView, so live→committed switches the data source, not the DOM.

Why

A live answer renders in the maka-turn-streaming section (chat-view.tsx); the settled answer renders from materializeTurns → TurnView. At text_complete the first subtree unmounts and the second mounts. Because they are two trees, any height or spacing difference between them lands as a visible jump the frame the stream settles.

PR #636's polish round removed that jump by hand-aligning the two trees: the streaming .maka-turn wrapper, margin-top: var(--space-3), the always-mounted status cluster with a :empty collapse, the equal-height footer placeholder, maka-footer-fade-in, and a 1000ms fallback-settle grace. That works, but it is a standing contract to keep two trees pixel-identical forever — this round alone had to reproduce and patch the jump twice (normal handoff, then thinking-only / textless turns). Each new element on either tree is a fresh regression surface.

First principles: the jump exists because of duplication. Removing the duplication makes it structurally impossible, and lets the hand-alignment special-cases be deleted.

Proposed approach

• Materialize the active streaming answer into a transient TurnViewModel / timeline item (thinking + text sourced from the in-memory streaming buffer, tools from live state) and render it through the same TurnView, or extract a shared TurnTimeline + TurnFooterSlot that both live and committed use.
• live→committed then swaps the data source on a stable DOM node: same TurnView, same Message, same footer slot. The text stops growing, the footer enables/fades in — nothing unmounts.
• Delete the streaming-only alignment special-cases once the seam is gone: streaming .maka-turn-streaming margin, footer placeholder, maka-footer-fade-in, and (if the seam no longer needs it) the fallback-settle grace.

Performance constraint (must not regress)

During plain-text streaming the hot path re-renders only the streaming bubble; materializeTurns output stays identity-stable for every committed turn, so the memoized TurnViews skip re-render (chat-view.tsx ~L298-305). A naive unification that re-materializes the whole turn list per frame would tank frame rate.

The unification must keep per-frame work to exactly the tail turn: materializeTurns must yield stable identities for all turns except the last, and only the tail turn's memo key tracks the streaming buffer. Acceptance must measure per-frame re-render count and prove it does not regress vs today's single-bubble cost.

Acceptance

• One render path: no separate maka-turn-streaming subtree; the streaming answer flows through TurnView.
• The hand-alignment special-cases above are removed.
• A height-neutral visual-smoke / contract test locks the seam (covering normal, thinking-only, and tool-only turn shapes).
• Per-frame re-render count during streaming is measured and shown not to regress.

Context

Follow-up from the PR #636 streaming-UI-rework polish round and its three-way review (ChatGPT / Codex / Pi), which converged on this as the durable fix. The polish commits (mono-ligature fix, tool trow rework, deep-thinking typography, single font tier, typewriter rAF-lag fix, height-neutral handoff, thinking-only footer reservation) ship in #636; this issue tracks the structural convergence deliberately deferred to its own PR because it touches the perf-critical render memoization and deserves independent review + perf verification.

Known residual folded in (from #636 three-way review)

Codex re-review of the #636 polish surfaced a residual the single-render-path fix must also close: the textless / thinking-only handoff is non-atomic. Text turns settle via the draining → settleAssistantStreaming handshake (refresh the committed assistant message BEFORE clearing the live slot), so the swap is atomic and the reserved footer box bridges it. But a thinking-only / textless completion (drainAssistantStreaming !applied.text in app-shell-session-events.ts, plus the abort-mid-think paths) clears the live section first and refreshes messages async — so the live subtree (and its footer placeholder) unmounts before the committed footer mounts, and the swap can still flash for that turn shape.

The footer placeholder groundwork shipped in #636 (chat-view.tsx, unconditional inside the live section) is correct but insufficient alone: it only makes the swap height-neutral when the live section is held to settle. Acceptance for this issue must cover the textless/thinking-only and abort-mid-think shapes with an atomic swap (no unmount-before-mount gap), so this specific residual is closed structurally rather than by adding another completion-path special-case.