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>

media/preview.ts

@@ -27,6 +27,7 @@ const summary = document.getElementById("cw-summary")!;
 const legend = document.getElementById("cw-legend")!;
 const annotationsEl = document.getElementById("cw-annotations") as HTMLInputElement | null;
 const pinEl = document.getElementById("cw-pin") as HTMLButtonElement | null;
+const askEl = document.getElementById("cw-ask") as HTMLButtonElement | null;
 
 // F10: the annotations on/off toggle.
 annotationsEl?.addEventListener("change", () => {
@@ -38,6 +39,13 @@ pinEl?.addEventListener("click", () => {
   vscodeApi.postMessage({ type: "pinBaseline" });
 });
 
+// F11 (SLICE-3): Ask Claude to Edit Document — post the document-scope intent; the
+// host prompts for the instruction + runs the turn (INV-8). SLICE-4 makes this
+// button adaptive (Edit Selection when text is selected in the preview).
+askEl?.addEventListener("click", () => {
+  vscodeApi.postMessage({ type: "askClaude", scope: "document" });
+});
+
 // F10: delegated ✓/✗ accept/reject of pending proposals (routed back to the F4 seam).
 body.addEventListener("click", (e) => {
   const btn = (e.target as HTMLElement)?.closest<HTMLElement>(".cw-actions button");