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>

package.json

@@ -78,6 +78,11 @@
         "command": "cowriting.showTrackChangesPreview",
         "title": "Cowriting: Open Review Preview",
         "category": "Cowriting"
+      },
+      {
+        "command": "cowriting.editDocument",
+        "title": "Ask Claude to Edit Document",
+        "category": "Cowriting"
       }
     ],
     "menus": {
@@ -101,6 +106,10 @@
         {
           "command": "cowriting.pinDiffBaseline",
           "when": "editorLangId == markdown"
+        },
+        {
+          "command": "cowriting.editDocument",
+          "when": "editorLangId == markdown"
         }
       ],
       "editor/context": [