feat(trace): unified per-conversation forensic recorder for chat + search

[quiet-node]

Summary

• Generalizes the search-pipeline trace recorder (PR feat(search): add forensic trace recorder #126) into a unified, per-conversation forensic recorder that captures every chat turn AND every search turn into JSONL files under app_data_dir()/traces/{chat,search}/<conversation_id>.jsonl.
• Single dev-only toggle: [debug] trace_enabled = true (also exposed as a Settings panel switch). Off by default. Legacy field search_trace_enabled continues to work via serde(alias).
• Output is dev-grade artifacts. Point an external agent (Claude Code, jq, a Python notebook) at the folder to study patterns and improve the system prompt. No in-app analyzer, no cloud upload, no fine-tune loop.

What changed

Backend (src-tauri/src/trace/) — new module:

• trace::ids::ConversationId newtype + new_turn_id() (moved from search/recorder.rs).
• trace::recorder::TraceRecorder trait (was PipelineRecorder), RecorderEvent enum (existing search variants unchanged + new chat variants: ConversationStart, UserMessage, AssistantThinking, AssistantTokens, AssistantComplete, ScreenCaptured, ConversationEnd), TraceDomain { Chat, Search }, FileRecorder::for_conversation(root, domain, conv_id), NoopRecorder, test-only MockRecorder.
• trace::registry::RegistryRecorder — production composition: parking_lot::RwLock<HashMap<(TraceDomain, ConversationId), Arc<FileRecorder>>> with lazy insert, hot-path Arc caching, ConversationEnd-triggered flush + evict, and append-mode late-event tolerance.
• trace::BoundRecorder — wraps Arc<dyn TraceRecorder> + ConversationId so existing recorder.record(event) callsites stay unchanged through the search pipeline.

Schema bump v1 → v2. Each JSONL line now carries top-level domain and conversation_id. Search-event variant names and fields are unchanged. File location moves from traces/turn-<id>.jsonl (one per search turn) to traces/{chat,search}/<conversation_id>.jsonl (one per conversation, per domain).

IPC contract changes. ask_ollama gains conversation_id: String, is_first_turn: bool, slash_command: Option<String>. search_pipeline gains conversation_id: String. capture_full_screen_command gains conversation_id: String. New record_conversation_end(conversation_id, reason) command lets the frontend signal user-perceived end-of-conversation (used by useOllama.reset() and useOllama.loadMessages()) so the chat-domain trace gets a clean closing line.

Frontend. useOllama mints a stable trace conversation id per session (lazy via ensureTraceConversationId), threads it through every invoke, fires record_conversation_end on reset() and loadMessages(). Settings UI label updated from "Search trace" → "Trace recording"; helper text reflects both domains.

Coverage gate. 100% lines + 100% functions across both Rust (cargo +nightly llvm-cov --fail-under-lines 100) and TypeScript (Vitest 100% threshold). 750 Rust tests + 1166 frontend tests all pass.

Design doc

~/.gstack/projects/quiet-node-thuki/logan-worktree-gentle-dancing-wave-design-20260506-135848.md (Status: APPROVED, post 2 rounds of adversarial spec review, 9/10 quality, 15 issues caught and fixed). Plus eng-review pass that surfaced the Settings-UI propagation scope.

Test plan

• bun run validate-build clean (zero warnings, zero errors)
• bun run test:all:coverage clean (100% line / function gate)
• Smoke test (Logan): edit config.toml, set [debug] trace_enabled = true, launch Thuki, run a conversation that includes /search and /screen, verify:
  • traces/chat/<id>.jsonl and traces/search/<id>.jsonl files appear
  • Chat file contains conversation_start, user_message, assistant_tokens, screen_captured, assistant_complete
  • Search file contains the existing search-pipeline events with the new conversation_id field
  • Reset conversation → conversation_end line appears with reason "user_reset"
  • Loading a saved conversation from history rotates the trace file (closes old + starts new)
  • Disabling the toggle silently switches the recorder back to noop on next launch
• Migration sanity (Logan): keep an existing [debug] search_trace_enabled = true line, launch, confirm serde(alias) accepts it (file is rewritten as trace_enabled only on next settings save)

Notes

• Single PR per request (combines what the design doc originally split into PR1 mechanical refactor + PR2 chat-domain capture).
• DO NOT MERGE until Logan's smoke test confirms the dev loop works end-to-end. Local /code-review will run before merge.