feat(f11): SLICE-3 — Edit Document button + per-hunk proposal path (#43, INV-37)

A whole-document Ask-Claude rewrite is diffed into hunks and surfaced as N
independent F4 proposals (one per changed hunk) — reusing the F4 single-range
model N times, no new model. Per spec §6.4/§7.2 SLICE-3.

- trackChangesModel: pure `diffToHunks(currentText, rewrittenText)` →
  EditHunk[] (vscode-free, deterministic; diffWordsWithSpace, coalescing
  adjacent add/remove runs; offsets index currentText).
- trackChangesPreview: `runEditAndPropose(document, target, instruction)` — the
  shared host routine (selection → one single-range propose; document → diff →
  one propose per hunk; never mutates the doc, INV-10); `askClaude` UI wrapper
  (host showInputBox keeps LLM/secrets out of the sealed webview, INV-8/35);
  injectable `editTurn` + `setEditTurnForTest` seam (no LLM in CI); the
  `askClaude` inbound message branch; `cowriting.editDocument` command for #42
  reuse.
- package.json: register cowriting.editDocument, palette-guarded on markdown.
- webview: ✦ Ask Claude to Edit Document button → { askClaude, scope:"document" }.
- unit: diffToHunks fixtures (zero/one/multi-hunk, wholesale, determinism).
- host E2E: stubbed multi-hunk rewrite → N matching proposals, doc untouched;
  editDocument command registered + markdown-guarded.

205 unit + 49 host E2E green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/trackChangesModel.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, test, expect } from "vitest";
-import { splitBlocks, splitBlocksWithRanges, diffBlocks, renderTrackChanges, colorByAuthor, type AuthorSpan } from "../src/trackChangesModel";
+import { splitBlocks, splitBlocksWithRanges, diffBlocks, diffToHunks, renderTrackChanges, colorByAuthor, type AuthorSpan } from "../src/trackChangesModel";
 
 describe("splitBlocks", () => {
   it("splits prose paragraphs on blank lines, dropping empties", () => {
@@ -320,6 +320,48 @@ describe("renderTrackChanges — intra-diagram mermaid (#22)", () => {
   });
 });
 
+// F11 SLICE-3 (INV-37, §6.4): a whole-document rewrite is diffed into per-hunk
+// proposal ranges — each an independent F4 single-range proposal. Pure,
+// vscode-free, deterministic; offsets index into currentText.
+describe("F11 diffToHunks (INV-37)", () => {
+  test("an identical rewrite → zero hunks", () => {
+    expect(diffToHunks("The same text.\n", "The same text.\n")).toEqual([]);
+  });
+
+  test("a single changed word → one hunk over exactly that word", () => {
+    const current = "The quick brown fox.";
+    const hunks = diffToHunks(current, "The quick red fox.");
+    expect(hunks).toHaveLength(1);
+    expect(current.slice(hunks[0].start, hunks[0].end)).toBe("brown");
+    expect(hunks[0].replacement).toBe("red");
+  });
+
+  test("two disjoint changes → two hunks with correct ranges + replacements", () => {
+    const current = "one two three four";
+    const hunks = diffToHunks(current, "one TWO three FOUR");
+    expect(hunks).toHaveLength(2);
+    expect(current.slice(hunks[0].start, hunks[0].end)).toBe("two");
+    expect(hunks[0].replacement).toBe("TWO");
+    expect(current.slice(hunks[1].start, hunks[1].end)).toBe("four");
+    expect(hunks[1].replacement).toBe("FOUR");
+    // disjoint + ordered
+    expect(hunks[0].end).toBeLessThanOrEqual(hunks[1].start);
+  });
+
+  test("a wholesale replacement (nothing in common) → one full-range hunk", () => {
+    const current = "alpha";
+    const hunks = diffToHunks(current, "omega");
+    expect(hunks).toHaveLength(1);
+    expect(hunks[0]).toEqual({ start: 0, end: current.length, replacement: "omega" });
+  });
+
+  test("is deterministic — same inputs → identical hunks", () => {
+    const a = diffToHunks("a b c d", "a B c D");
+    const b = diffToHunks("a b c d", "a B c D");
+    expect(a).toEqual(b);
+  });
+});
+
 // F11 SLICE-2 (INV-36): the pure render layer emits data-src-start/data-src-end
 // (source char offsets from BlockWithRange) on every LIVE-source rendered block,
 // in BOTH modes. The webview's selection→source mapping walks the DOM to the