Phase 4: Ask Petal SSE chat

Conversational follow-up on a suggestion, streamed token-by-token.

Backend (interface-only; handlers never touch a concrete LLM client):
- internal/llm/chat.go: StreamAskPetal with conversational sampling
  (max_tokens 512, temp 0.7, rep 1.15, top_p 0.92, stop "\n\n\n"),
  reusing AskPetalSystemPrompt + TrimHistory.
- internal/suggestions/chat.go: POST /api/suggestions/:id/chat. One
  user-scoped join loads the suggestion + parent content_text;
  surroundingParagraph extracts the \n\n-bounded paragraph at from_pos
  (whole-doc fallback when unlocated) and injects it server-side.
  Streams event: token / event: done SSE frames with JSON-encoded data
  so token newlines can't break framing; real http.Flusher per chunk.
  LLM-unreachable -> 502 before SSE headers; unknown suggestion -> 404.

Frontend:
- streamSuggestionChat: fetch + ReadableStream SSE parser (not
  EventSource, needs POST), abortable.
- AskPetal.tsx: whole conversation in component state (no persistence,
  cleared on close), Petal's first bubble pre-seeded with the
  explanation, rose/lavender bubbles, CJK font stack on the bubbles
  only (Note #17), streaming caret.
- SuggestionCard "Ask Petal" pill pins the card open while chatting
  (hover-close suppressed, click-away closes) and widens it to 340px.

Tests: chat_test.go covers streamed-text concat + done event,
server-side context injection on the system message, sampling params,
404, and surroundingParagraph. go build/vet/test clean, tsc clean,
vite build OK. Live SSE smoke-tested against a fake streaming vLLM:
tokens flushed individually through the chi middleware stack, done
terminator, 502 on LLM-down, 404 on unknown suggestion.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

BUILD_PLAN.md

@@ -46,10 +46,11 @@ Multi-session build. **Source of truth for what's done and what's next.** Update
 - [x] `SuggestionHighlight` (ProseMirror **decorations**, re-anchored by `original` string on every doc change — not stored marks) + `SuggestionCard` (accept applies replacement in-editor then PATCHes; dismiss)
 - [x] Suggestion colors: grammar=mint, phrasing=peach, idiom=lavender, clarity=sky (honey reserved for voice)
 
-### Phase 4 — Ask Petal (conversational follow-up)
-- [ ] `POST /api/suggestions/:id/chat` SSE streaming; server-side context injection
-- [ ] AskPetal component, token-by-token render, no persistence
-- [ ] CJK font fallbacks on chat bubbles (spec Note #17)
+### Phase 4 — Ask Petal (conversational follow-up) ✅
+- [x] `POST /api/suggestions/:id/chat` SSE streaming; server-side context injection — `internal/suggestions/chat.go` loads the suggestion + parent doc in one user-scoped query, extracts the `\n\n`-bounded paragraph around `from_pos` (falls back to truncated doc when `from_pos == -1`), injects it via `AskPetalSystemPrompt`, streams `event: token`/`event: done` SSE frames (JSON-encoded data so token newlines don't break framing). LLM-unreachable returns a clean 502 before any SSE headers; unknown suggestion 404s.
+- [x] AskPetal component, token-by-token render, no persistence — `AskPetal.tsx` holds the whole conversation in component state (cleared on close), pre-seeds Petal's first bubble with the suggestion explanation, streams via `streamSuggestionChat` (fetch + ReadableStream, not EventSource). `SuggestionCard` gains an "Ask Petal ✨" pill; the card pins open (hover-close suppressed, click-away to dismiss) while the panel is expanded.
+- [x] CJK font fallbacks on chat bubbles (spec Note #17) — bubbles + input use the `'Nunito','PingFang SC','Microsoft YaHei','Noto Sans CJK SC'` stack (the user asks in Mandarin); applied to the chat surface only, not the serif editor body.
+- `internal/llm/chat.go`: `StreamAskPetal` (max_tokens 512, temp 0.7, rep 1.15, top_p 0.92, stop `\n\n\n`) reusing the existing `AskPetalSystemPrompt` + `TrimHistory`. Backend stays interface-only; the SSE handler never touches a concrete client.
 
 ### Phase 5 — Voice consistency pass (Tier 1)
 - [ ] `POST /api/docs/:id/voice`, whole-document, slow cadence / explicit action
@@ -75,3 +76,4 @@ Multi-session build. **Source of truth for what's done and what's next.** Update
 - 2026-06-25: **Phase 1 complete.** `internal/db` package: modernc.org/sqlite (pulled go toolchain → 1.25), `Open()` does mkdir + WAL/foreign-keys DSN + versioned migration runner + idempotent local-user seed. Models with type/status constants. Wired into `main.go`; tests pass (migrate/seed idempotency, CHECK reject, FK cascade). Verified server boots and writes `petal.db`. Next: **Phase 2 (document CRUD + auto-save)** — first "it works" milestone.
 - 2026-06-25: **Phase 2 complete.** Backend `internal/docs`: chi sub-router (list/create/get/update/delete) mounted at `/api/docs`, local-user scoped, RETURNING on create, COALESCE partial-update (one PUT serves rename + full save), 404/400 JSON errors; `handlers_test.go` walks the full lifecycle. Frontend: `api/client.ts`, `useAutoSave` (1.5s debounce + `saveNow` flush), `EditorCore` (Tiptap StarterKit/Underline/TextAlign/Placeholder/CharacterCount) + `Toolbar`, `DocList`/`DocListItem`, `StatusBar`, rewritten `App.tsx` orchestrating load/select/create/delete with optimistic sidebar patching. `.petal-prose` styles (Lora body, Nunito headings). tsc clean, vite build OK, go build OK; smoke-tested full CRUD incl. CJK title round-trip + SPA serve. Next: **Phase 3 (LLM grammar checkpoint).**
 - 2026-06-25: **Phase 3 complete.** Backend `internal/llm`: `LLMClient` interface + factory (vLLM OpenAI-compat + Ollama native, both Complete/Stream), `prompts.go` (checkpoint + Ask Petal templates), `checkpoint.go` (brace-matched JSON salvage, per-doc 30s `RateLimiter`, doc/history truncation). `internal/suggestions`: `/api/docs/:id/check` + `:id/suggestions` + `/api/suggestions/:id/{accept,dismiss}`; each check replaces the pending set in a tx (accepted/rejected kept as history), throttled checks return the current set, positions located by `strings.Index` (advisory only). Frontend: `useCheckpoint` (4s debounce, loads existing on doc open, run-token guards stale responses), `SuggestionHighlight` Tiptap extension rendering ProseMirror **decorations** re-anchored by `original` string on every doc change (precise textblock offset→PM-pos mapping, handles inline atoms), `SuggestionCard` (type-colored tag, original→replacement diff, accept applies replacement in-editor + PATCHes, hover-bridge with close delay), breathing rose checkpoint dot in StatusBar, suggestion fade-float + breathe CSS. Tests: llm parse/rate-limit/truncate, suggestions full flow + rate-limit over httptest with a stub client. go build/vet/test clean, tsc clean, vite build OK; end-to-end smoke-tested against a fake vLLM endpoint (anchoring verified: `I has`→0:5, `two apple`→6:15) and 502 path when LLM unreachable. Next: **Phase 4 (Ask Petal SSE chat).**
+- 2026-06-25: **Phase 4 complete.** Backend: `internal/llm/chat.go` (`StreamAskPetal` — conversational sampling params, reuses `AskPetalSystemPrompt`/`TrimHistory`), `internal/suggestions/chat.go` (`POST /api/suggestions/:id/chat` — one user-scoped join loads the suggestion + parent `content_text`, `surroundingParagraph` extracts the `\n\n`-bounded paragraph at `from_pos` with whole-doc fallback, streams `event: token`/`event: done` SSE frames with JSON-encoded data, `X-Accel-Buffering: no`, real `http.Flusher` per chunk; LLM-down → 502 before SSE headers, unknown id → 404). Handler imports the interface only. Frontend: `streamSuggestionChat` (fetch + ReadableStream SSE parser, abortable), `AskPetal.tsx` (in-component history — no persistence, pre-seeded first bubble, rose/lavender bubbles, CJK font stack per Note #17, streaming caret), `SuggestionCard` "Ask Petal ✨" pill that pins the card open (hover-close suppressed, click-away closes) and widens it to 340px. Tests: `chat_test.go` (streamed-text concat + done event, server-side context injection asserted on the system message, sampling params, 404, `surroundingParagraph` unit). go build/vet/test clean, tsc clean, vite build OK. Live SSE smoke test against a fake streaming vLLM (fresh ports 8077/8088 — a pre-existing dev petal on :8099 left untouched): tokens flushed individually through the chi middleware stack, `done` terminator, 502 on LLM-down, 404 on unknown suggestion all verified. Next: **Phase 5 (voice consistency pass, Tier 1).**