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>

src/trackChangesModel.ts

@@ -9,7 +9,7 @@
  * DOM — `markdown-it` and `diff` are libraries; mermaid runs later in the webview.
  */
 import MarkdownIt from "markdown-it";
-import { diffArrays, diffWords } from "diff";
+import { diffArrays, diffWords, diffWordsWithSpace } from "diff";
 import { diffMermaid } from "./mermaidDiff";
 
 export type BlockType = "prose" | "code" | "mermaid";
@@ -193,6 +193,50 @@ export function diffBlocks(baselineText: string, currentText: string): BlockOp[]
   return ops;
 }
 
+/** A contiguous changed region of `currentText` and its replacement (F11). */
+export interface EditHunk {
+  /** char offset of the hunk's first changed char in currentText. */
+  start: number;
+  /** char offset one past the hunk's last changed char (start==end → a pure insertion). */
+  end: number;
+  /** the text that replaces currentText.slice(start, end). */
+  replacement: string;
+}
+
+/**
+ * F11 (INV-37, §6.4): diff a whole-document rewrite into per-hunk replacement
+ * ranges — each becomes one independent F4 single-range proposal (so a document
+ * edit surfaces as N independently ✓/✗-able blue blocks, no new model). Pure,
+ * vscode-free, deterministic. Offsets index into `currentText`: removed +
+ * unchanged parts reconstruct it exactly, so advancing on those yields true
+ * source offsets. Adjacent added/removed runs coalesce into one hunk; an
+ * unchanged part flushes the open hunk.
+ */
+export function diffToHunks(currentText: string, rewrittenText: string): EditHunk[] {
+  const hunks: EditHunk[] = [];
+  let offset = 0;
+  let open: EditHunk | null = null;
+  const flush = () => {
+    if (open) hunks.push(open);
+    open = null;
+  };
+  for (const part of diffWordsWithSpace(currentText, rewrittenText)) {
+    if (part.added) {
+      open ??= { start: offset, end: offset, replacement: "" };
+      open.replacement += part.value;
+    } else if (part.removed) {
+      open ??= { start: offset, end: offset, replacement: "" };
+      offset += part.value.length;
+      open.end = offset;
+    } else {
+      flush();
+      offset += part.value.length;
+    }
+  }
+  flush();
+  return hunks;
+}
+
 const md = new MarkdownIt({ html: true, linkify: false, breaks: false });
 // mermaid fences → <pre class="mermaid">SRC</pre> for client-side rendering; all
 // other fences fall through to markdown-it's default (escaped <pre><code>).