feat(phase8 #77 D8.4b): FE message-parts renderer (text/tool/source/citation/activity)

[earayu]

Summary

D8.4b first-cut. Replaces the legacy `AgentTurnCard` + `legacy-snapshot-shim` projection with a renderer that consumes the new `useAgentTurnStream` seam (D8.4a, merge `63a9d522`) directly. Each `AgentMessagePart` is rendered by type; transient `data-activity` is surfaced through a separate inline indicator and never persisted. Slot props `ConsentSlot` / `ElicitationSlot` carry the only handoff into #78.

What lands

File	Change
`web/src/components/chat/agent-turn-renderer.tsx`	NEW ~700 LOC. Activity stream + answer + references + slots all derive from the `parts` array.
`web/src/components/chat/agent-turn-card.tsx`	DELETE (1279 LOC).
`web/src/features/agent-runtime/legacy-snapshot-shim.ts`	DELETE; index re-exports updated.
`web/src/components/chat/chat-messages.tsx`	`AgentTurnStreamCard` now feeds the hook output directly into `AgentTurnRenderer`; `projectToLegacySnapshot` projection layer is gone.
`web/src/i18n/{zh-CN,en-US}/page_chat.json` (+ aggregated catalogs)	Adds `activity_stream.tool.{title,state.}`, `activity_stream.transient.`, `activity_stream.consent.placeholder_*`, `activity_stream.elicitation.placeholder_state`, `activity_stream.{completed_empty,pending_empty}`, `answer_section.completed_empty`.

Slot props (the only seam crossing into #78 territory)

```ts
type ConsentSlotProps = {
chatId: string;
turnId: string;
part: AgentToolConsentPart;
};
type ElicitationSlotProps = {
chatId: string;
turnId: string;
part: AgentElicitationPart;
};

type AgentTurnRendererProps = {
// ... part stream + status from useAgentTurnStream
ConsentSlot?: React.ComponentType;
ElicitationSlot?: React.ComponentType;
};
```

#78 chenyexuan implements `consent-prompt.tsx` + `elicitation-form.tsx` that conform to these prop signatures; both call `decideToolConsent` / `submitElicitation` from the agent-runtime API client landed in D8.4a. The slot props are optional by design — the placeholder fallback (`` / ``) keeps the parts visible even before #78 lands.

Visual baseline preserved

The L1 design and existing card affordance is intact:

• avatar + status badge + timestamp header,
• activity stream Collapsible (now: tool calls + consent/elicitation slots + transient activity indicator),
• answer Card / streaming markdown,
• debug Collapsible (turn id / request id / status / error code+message),
• references Sheet (now sourced from `source-url` / `source-document` / `data-citation` parts),
• feedback + copy.

Hook seam contract — unchanged

`useAgentTurnStream(...) → { parts, transientActivity, status, errorText, lastSequence, abort }` is consumed verbatim. No transport-layer changes; this PR is purely the rendering layer.

Verification

• `yarn lint` clean.
• `tsc --noEmit` clean for the touched files (the four pre-existing errors in `chat-input.tsx` are unrelated and untouched here).
• `yarn dev` boots in 2.8s on port 3012; `GET /`, `/auth/signin`, `/workspace/collections`, `/workspace` all return 200.

Out of scope

• EOF-before-terminal regression test (Weston msg=b7ae3bfd D8.4a follow-up) is not bundled here — there is no FE test infra in the repo today, and adding `vitest` is its own scope. Behavior is documented at the relevant code paths in `stream-client.ts` + `reducer.ts`; recommend a dedicated test-infra PR after feat: support default collections #77/chore: fix embedding timeout and scale the embedding workers #78 land.
• Interactive consent / elicitation bodies — owned by chore: fix embedding timeout and scale the embedding workers #78 chenyexuan via the slot props.

Test plan

• Symphony architect canonical drift quick check on the parts-to-render mapping (esp. tool state lifecycle + citation/source aggregation rules)
• Weston minimal CR — confirm renderer does not regress hook contract / transport boundary
• dongdong scoped FE seam review — confirm slot props interface is stable for chore: fix embedding timeout and scale the embedding workers #78 to plug into without renderer changes
• Manual visual smoke once a fresh tenant is available (run an agent turn → confirm activity stream / streaming answer / references all render from `parts`)
• Confirm consent / elicitation placeholder fallback renders when chore: fix embedding timeout and scale the embedding workers #78 has not landed yet

Hand-off seam for #78

• Branch `fe/d8.4b-parts-renderer` head `b532abcd`; chenyexuan can base off this commit and add `consent-prompt.tsx` + `elicitation-form.tsx` that match `ConsentSlotProps` / `ElicitationSlotProps`.
• Wiring is a one-line change in `chat-messages.tsx`'s `AgentTurnStreamCard` (`<AgentTurnRenderer ... ConsentSlot={ConsentPrompt} ElicitationSlot={ElicitationForm} />`).

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