## Why

Stage one removed final-answer fields from Executor, stage two added deterministic Gatekeeper checks, and stage three made Verifier claim-oriented. The remaining gap is final answer rendering: ChatService can still rely on temporary V2 rendering paths instead of a dedicated expression layer, which risks letting unverified Executor material shape user-facing answers.

This phase introduces a Composer expression layer so final Chat answers are generated only from Verifier-allowed material.

## What Changes

- Add a `chat_composer` final-answer generation step after Verifier.
- Add a strict Composer prompt and JSON output contract with `answer_summary`, `recommended_actions`, and `user_facing_answer`.
- Build Composer input in `ChatService` from filtered Verifier results:
  - `allowed_claims`
  - `allowed_hypotheses`
  - `missing_info`
  - `recommended_actions`
  - `rationale`
- Ensure PASS, LOW_CONFID, and REJECT final answers no longer read Executor `user_facing_answer` or raw Executor JSON.
- Persist Composer output in `diagnosis_session.self_evaluation.verifier_evaluation.composer_output`.
- Add safe fallback templates for malformed Composer output that never expose raw Executor JSON or raw Composer JSON.

## Capabilities

### New Capabilities

- `chat-composer-agent`: Final expression layer that turns Verifier-allowed structured material into a readable Chinese user answer without introducing new facts.

### Modified Capabilities

- `chat-verifier-agent`: ChatService final routing changes so Verifier verdicts feed Composer or safe fixed templates instead of direct Executor answer paths.

## Impact

- Affected prompt: add `src/main/resources/prompts/chat-composer-prompt.md`.
- Affected service: `ChatService` final rendering after Verifier, Composer input assembly, Composer output parsing, malformed-output fallback.
- Affected audit: `diagnosis_session.self_evaluation.verifier_evaluation` gains `composer_output`.
- Affected tests: `ChatServiceSequentialAgentTest` and focused tests around Composer input filtering, final answer routing, and malformed Composer fallback.
- Database schema: no table or column change.
- External API: no endpoint contract change; final answer text semantics become stricter because unverified Executor material is no longer a source for user-visible answers.

## Non-Goals

- No Planner changes.
- No Gatekeeper rule expansion.
- No Verifier classification changes.
- No Executor retry behavior change.
- No database migration.
- No stage-five eval fixture expansion in this phase.