Goal

Rebuild the session message flow so the timeline can be shipped incrementally behind the perf gate instead of through another large slice like PR #589.

Scope

In scope:

• User message bubbles.
• Agent prose and reasoning output.
• Tool call trows in collapsed and expanded states.
• Inline copy buttons.
• Attachment chips.
• Interrupted and system events.
• Reset / Fork toolbar entries.
• Streaming state such as work timer, thinking label, and animation timing.

Out of scope:

• Right panel work from [Task] UI rewrite v2 Area B: right panel modular rollout #602.
• Home work from [Task] UI rewrite v2 Area C: home surface modular rollout #603.
• Visual shell or final shell work from [Feature] UI rewrite v2 Area E1: visual shell modular rollout #605 and [Task] UI rewrite v2 Area E2: final shell modular rollout #606.
• Any implementation before [Task] UI rewrite v2 PR0: perf-gated regression CI #600 lands.

Relevant files or context

Parent: #599

Audit-first file set:

• packages/ui/src/components/message-part.tsx
• packages/app/src/pages/session/message-timeline.tsx
• packages/ui/src/components/session-turn.tsx
• packages/app/src/pages/session.tsx
• packages/app/src/pages/session/use-session-commands.tsx
• packages/ui/src/components/markdown.tsx (decide whether this area still needs to touch it; default defer)
• packages/app/src/pages/session/session-timeline-scroll-controller.ts
• packages/app/src/pages/session/use-session-history-window.ts
• packages/app/src/pages/session/use-session-followups.ts

Constraints carried from the master plan:

• PR0 perf gate must be green on every PR.
• Do not regress PR feat(ui,app): sidebar L35 row rewrite (slice 09, issue #440) #495 or PR feat(ui,app): composer rewrite (slice 10, issue #440) #508.
• Soft file-size rule: 500 LOC, ideal 200 LOC, no splitting for the sake of splitting.

Verification

• Every Area A PR passes [Task] UI rewrite v2 PR0: perf-gated regression CI #600.
• The area-specific read-only audit exists before the first code PR.
• The 5-minute visual pass in docs/design/session-view-w1-manual-checklist.md passes.
• W1 e2e coverage in the [Task] chore(e2e): strengthen W1 regression coverage #598 family remains green.

Execution mode

Agent should investigate and propose a plan first