Stop regenerating the world on every check

A card vanishing and coming back seconds later, with different words, was
never about latency: every pass deleted its whole family and re-inserted
it, so each round minted new row ids. The rail keys on suggestion.id, so a
full remount was guaranteed — new id, new created_at (hence the re-fired
chime), and a fresh explanation from a model that re-reasons every time it
is asked. One unchanged mistake carried three different explanations in a
single sitting.

Passes now reconcile instead of replace. A re-proposed edit keeps its row:
its id, its created_at, and the wording she has already read. And the
grammar checkpoint stops asking about sentences nobody touched — the
document is split into hashed sentences, checked_chunks records which ones
a family has read, and only the difference is sent. When nothing changed
it doesn't call the model at all, and doesn't spend its rate-limit slot on
having done nothing.

The tone is part of a sentence's identity: cached advice was written for
the old register, so switching doc type re-reads every line.

replaceMechanics reconciles too, which mattered more than expected — the
rule pack fires 250 ms after a keystroke, so it was re-minting every local
card's id several times a sentence.

Only the grammar checkpoint is chunked. Voice is a property of the whole
document, and the collocation coach is a button she pressed asking for a
fresh read.

No client change was needed; stable ids were the whole of it.

Claude-Session: https://claude.ai/code/session_016y6gyuHkQXPiEuW8RGQyua

internal/suggestions/handlers_test.go

@@ -20,10 +20,17 @@ import (
 type stubClient struct {
 	response string
 	calls    int
+	// The full prompt of the most recent call, so a test can assert which
+	// sentences a chunked pass actually asked about.
+	lastPrompt string
 }
 
-func (s *stubClient) Complete(_ context.Context, _ llm.CompletionRequest) (string, error) {
+func (s *stubClient) Complete(_ context.Context, req llm.CompletionRequest) (string, error) {
 	s.calls++
+	s.lastPrompt = ""
+	for _, m := range req.Messages {
+		s.lastPrompt += m.Content + "\n"
+	}
 	return s.response, nil
 }
 
@@ -64,6 +71,17 @@ func newTestServer(t *testing.T, client llm.LLMClient) (http.Handler, string, *H
 	return authed, docID, h
 }
 
+// setDocText rewrites the seeded document, standing in for the writer editing.
+// The grammar checkpoint only asks the model about sentences that changed since
+// it last read the document, so a test that wants a second real pass has to
+// change something first — as she always has.
+func setDocText(t *testing.T, h *Handler, docID, text string) {
+	t.Helper()
+	if _, err := h.DB.Exec(`UPDATE documents SET content_text = ? WHERE id = ?`, text, docID); err != nil {
+		t.Fatalf("update doc text: %v", err)
+	}
+}
+
 func do(t *testing.T, srv http.Handler, method, path, body string) *httptest.ResponseRecorder {
 	t.Helper()
 	var r *http.Request
@@ -183,6 +201,7 @@ func TestFickleEditsSuppressed(t *testing.T) {
 	]}`}
 	srv, docID, h := newTestServer(t, client)
 	h.Limit = llm.NewRateLimiter(0)
+	setDocText(t, h, docID, `He left "early," because of the rain. The cat always have a calm face.`)
 
 	rec := do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
 	var got []db.Suggestion
@@ -194,6 +213,10 @@ func TestFickleEditsSuppressed(t *testing.T) {
 		do(t, srv, http.MethodPost, "/suggestions/"+s.ID+"/accept", "")
 	}
 
+	// Both edits are now in the document, which is what re-opens those sentences
+	// for a second reading.
+	setDocText(t, h, docID, `He left "early," due to the rain. The cat always has a calm face.`)
+
 	// Reversal of the first accept (note the " → ' quote churn) and a re-polish of
 	// the second accept must both be dropped; only the unrelated edit survives.
 	client.response = `{"suggestions":[
@@ -314,7 +337,9 @@ func TestCollocationPassCoexists(t *testing.T) {
 		t.Fatalf("collocation response should carry all three families, got %+v", got)
 	}
 
-	// A grammar checkpoint must NOT wipe the voice or collocation flags.
+	// A grammar checkpoint must NOT wipe the voice or collocation flags. She fixes
+	// the flagged sentence, so its own grammar row goes and nothing replaces it.
+	setDocText(t, h, docID, "I have two apples.")
 	client.response = `{"suggestions":[]}`
 	rec = do(t, srv, http.MethodPost, "/docs/"+docID+"/check", "")
 	if rec.Code != http.StatusOK {