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/chunk_test.go

@@ -0,0 +1,118 @@
+package suggestions
+
+import (
+	"strings"
+	"testing"
+)
+
+func texts(chunks []chunk) []string {
+	out := make([]string, 0, len(chunks))
+	for _, c := range chunks {
+		out = append(out, strings.TrimSpace(c.text))
+	}
+	return out
+}
+
+func TestSplitChunks(t *testing.T) {
+	cases := []struct {
+		name string
+		in   string
+		want []string
+	}{
+		{
+			name: "plain sentences",
+			in:   "I has two apple. She go to market yesterday! Why?",
+			want: []string{"I has two apple.", "She go to market yesterday!", "Why?"},
+		},
+		{
+			// A decimal must not split, or the sentence's identity would churn
+			// while she types the number.
+			name: "decimals stay whole",
+			in:   "It costs 3.50 today. Tomorrow, more.",
+			want: []string{"It costs 3.50 today.", "Tomorrow, more."},
+		},
+		{
+			name: "closing quote travels with its sentence",
+			in:   `He said "early," and left. She stayed.`,
+			want: []string{`He said "early," and left.`, "She stayed."},
+		},
+		{
+			// Chinese runs sentences together with no space after 。 — she writes
+			// in both languages in one document.
+			name: "cjk terminators split without a space",
+			in:   "我想说这句话。但是不知道用英语怎么说。",
+			want: []string{"我想说这句话。", "但是不知道用英语怎么说。"},
+		},
+		{
+			name: "newlines break chunks",
+			in:   "A list item\nAnother item\n",
+			want: []string{"A list item", "Another item"},
+		},
+		{
+			name: "blank runs are dropped",
+			in:   "\n\n  \nOnly this.\n\n",
+			want: []string{"Only this."},
+		},
+		{
+			name: "trailing fragment is its own chunk",
+			in:   "Done. Still writing",
+			want: []string{"Done.", "Still writing"},
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := texts(splitChunks(tc.in, ""))
+			if len(got) != len(tc.want) {
+				t.Fatalf("want %q, got %q", tc.want, got)
+			}
+			for i := range got {
+				if got[i] != tc.want[i] {
+					t.Fatalf("chunk %d: want %q, got %q", i, tc.want[i], got[i])
+				}
+			}
+		})
+	}
+}
+
+// A sentence's identity survives the churn that doesn't change what it says:
+// the editor rewrites quotes as she types, and a paragraph reflows.
+func TestChunkIdentityIgnoresCosmeticChurn(t *testing.T) {
+	a := splitChunks(`She said "hello" softly.`, "")
+	b := splitChunks("She said “hello”   softly.", "")
+	if len(a) != 1 || len(b) != 1 {
+		t.Fatalf("want one chunk each, got %d and %d", len(a), len(b))
+	}
+	if a[0].hash != b[0].hash {
+		t.Fatalf("quote/whitespace churn changed the sentence's identity")
+	}
+	if same := splitChunks(`She said "hello" softly.`, "academic"); same[0].hash == a[0].hash {
+		t.Fatalf("a different tone must be a different reading of the sentence")
+	}
+}
+
+func TestChangedChunksAndLookup(t *testing.T) {
+	chunks := splitChunks("One thing. Another thing. One thing.", "")
+	if len(chunks) != 3 {
+		t.Fatalf("want 3 chunks, got %d", len(chunks))
+	}
+
+	// A repeated sentence is one question, not two.
+	if got := changedChunks(chunks, nil); len(got) != 2 {
+		t.Fatalf("want 2 distinct changed chunks, got %d", len(got))
+	}
+
+	checked := hashSet(chunks[:1])
+	changed := changedChunks(chunks, checked)
+	if len(changed) != 1 || strings.TrimSpace(changed[0].text) != "Another thing." {
+		t.Fatalf("want only the unread sentence, got %q", texts(changed))
+	}
+
+	if chunkFor("Another", chunks) != chunks[1].hash {
+		t.Fatalf("span was attributed to the wrong sentence")
+	}
+	// A span the document doesn't contain has no sentence, so it is never cached.
+	if chunkFor("nowhere in here", chunks) != "" {
+		t.Fatalf("unanchorable span should have no chunk")
+	}
+}