Phase 12 + 13: collocation coach + vocabulary garden

Phase 12 — collocation coach: a third suggestion family for gentle
"natives usually say…" hints on non-native word pairings, reusing the
existing runPass/pendingScope/rail machinery.
- llm/collocation.go (RunCollocation, 25s floor, reuses ParseCheckpoint)
  + collocationSystemPrompt/CollocationMessages (warm, Mandarin gloss,
  defers grammar/spelling to the grammar family)
- migration 0005 rebuilds the suggestions table to extend the type CHECK
  (SQLite can't ALTER a CHECK)
- collocationScope + CollocationLimit + POST /{id}/collocation
- fix: grammarScope was `type != 'voice'` and would wipe the new
  collocation flags; now `type NOT IN ('voice','collocation')`
- frontend: --color-blossom, "Make it sound natural 🌸" pill,
  collocating/runCollocation in useCheckpoint, StatusBar dot

Phase 13 — vocabulary garden: capture looked-up words and surface them
for gentle spaced repetition.
- new internal/vocab package: migration 0006 (vocab_words, SM-2-lite
  columns, doc_id ON DELETE SET NULL, UNIQUE(user_id,word)),
  scheduler.go (Leitner ladder 1/3/7/16/35 then geometric; gentle
  "again", no streak-shaming), handlers (capture-upsert/list/due/
  review/delete, owner-scoped, SQLite-side datetime math)
- auto-capture on word lookup (dictionary-known words only, captures
  the surrounding sentence + doc_id) + 🤍/💚 toggle on WordCard
- GardenPanel: blossom grid (bloom by reps), flashcard review (sentence
  blanked, flip, again/good/easy, recognition↔production), sleepy-kitten
  footer; opened from a global 🌷 header button

Tests: TestCollocationPassCoexists, vocab scheduler + handlers, db CHECK
extended. go build/vet/test + tsc + vite + vitest (51/51) clean;
migration verified against a copy of the live DB; live backend smoke
walked the full vocab lifecycle + the warm-502 collocation path.

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

internal/llm/collocation.go

@@ -0,0 +1,35 @@
+package llm
+
+import (
+	"context"
+	"time"
+)
+
+// CollocationInterval is the minimum gap between collocation passes for one
+// document. Like the voice pass it runs on an explicit user action ("Make it
+// sound natural") rather than a typing cadence, so this floor only guards the
+// inference endpoint against the button being mashed.
+const CollocationInterval = 25 * time.Second
+
+// RunCollocation sends the WHOLE document for a collocation pass — gentle
+// "natives usually say…" hints on non-native word pairings — and parses the JSON
+// result with the checkpoint's tolerant parser. It is deliberately NOT
+// TruncateDoc'd: like the voice pass it reads the whole document so the hints
+// reflect the full piece. Each flag carries a native replacement to apply.
+//
+// The tone argument is accepted for a uniform pass signature and passed through
+// to the prompt so a hint can prefer a register-appropriate pairing.
+func RunCollocation(ctx context.Context, client LLMClient, contentText, tone string) ([]RawSuggestion, error) {
+	raw, err := client.Complete(ctx, CompletionRequest{
+		Messages:          CollocationMessages(contentText, tone),
+		MaxTokens:         2048,
+		Temperature:       0.3,
+		RepetitionPenalty: 1.15,
+		TopP:              0.9,
+		Stop:              []string{"```", "\n\n\n\n"},
+	})
+	if err != nil {
+		return nil, err
+	}
+	return ParseCheckpoint(raw)
+}

internal/llm/prompts.go

@@ -92,6 +92,54 @@ func VoiceMessages(contentText string) []Message {
 	}
 }
 
+// collocationSystemPrompt drives the collocation coach — the single most
+// valuable polish for an ESL writer. It flags word PAIRINGS that are not wrong,
+// just non-native ("do a decision" → "make a decision", "strong rain" → "heavy
+// rain"), and explicitly DEFERS real grammar/spelling errors to the grammar
+// checkpoint so the two families don't overlap. Every explanation is framed as a
+// warm "natives usually say…" note with a short Mandarin gloss — never
+// "error/wrong" — because these are stylistic, not mistakes. It is a distinct
+// pass from the grammar checkpoint (do not bundle them). `replacement` carries
+// the natural pairing the writer can accept in one tap.
+const collocationSystemPrompt = `You are a warm, encouraging writing assistant helping someone who speaks English as a second language. ` +
+	`You are reviewing a COMPLETE document for COLLOCATIONS only — the natural word pairings native speakers use.
+
+A collocation is a pair or short group of words that native speakers habitually say together. ESL writers ` +
+	`often choose words that are grammatically correct but sound non-native: "do a decision" instead of "make a decision", ` +
+	`"strong rain" instead of "heavy rain", "say a joke" instead of "tell a joke". These are NOT grammar mistakes — they ` +
+	`are just not what a native speaker would naturally say.
+
+Identify up to 5 such non-native word pairings. For each, give the natural pairing a native speaker would use. ` +
+	`Be gentle and specific. Do NOT flag grammar errors, spelling mistakes, or unclear sentences — those are handled ` +
+	`elsewhere. Only flag word pairings that are correct but sound non-native.%s
+
+Phrase every explanation warmly as "Natives usually say…" and include a brief Simplified Chinese gloss in parentheses. ` +
+	`Never use the words "error", "wrong", or "mistake" — these are friendly polish, not corrections.
+
+Respond ONLY with valid JSON. No preamble, no markdown fences. Format:
+{
+  "suggestions": [
+    {
+      "original": "exact word pairing from the document",
+      "replacement": "the natural native pairing",
+      "explanation": "friendly note, e.g. 'Natives usually say \"make a decision\" rather than \"do a decision\" (native usage / 地道说法).'",
+      "type": "collocation"
+    }
+  ]
+}
+
+If every pairing already sounds natural, return: {"suggestions": []}`
+
+// CollocationMessages builds the message array for a collocation pass over the
+// WHOLE document (no truncation), gently steered toward the document's tone so a
+// hint can prefer a register-appropriate pairing.
+func CollocationMessages(contentText, tone string) []Message {
+	return []Message{
+		{Role: "system", Content: fmt.Sprintf(collocationSystemPrompt, toneGuidance(tone))},
+		{Role: "user", Content: contentText},
+	}
+}
+
 // askPetalSystemTemplate is the Ask Petal tutor prompt. The suggestion context
 // is interpolated in; the user's own messages are appended after this system
 // turn by the caller.