feat(f11): SLICE-5 — gateway, non-authorable disabling, docs (#43)

Closes the F11 feature: makes the toolbar surface reachable end to end and
guards the edit controls. Per spec §6.4/§6.5/§7.2 SLICE-5.

- package.json: cowriting.showTrackChangesPreview added to editor/title
  (when: editorLangId == markdown) — the minimal right-click → Open Review
  Preview gateway (#41/#42 expand it later).
- trackChangesPreview: the gateway command accepts the tab's resource Uri
  (palette/keybinding still fall back to the active editor); refresh() sends an
  `authorable` flag on both render messages; `editControlsEnabled` test seam.
- webview: disable Pin + Ask-Claude on a non-authorable doc (Annotations stays
  active — reading is always allowed); RenderMessage.authorable.
- host E2E: the editor/title gateway opens the preview + is markdown-guarded;
  edit controls disabled on a non-authorable (read-only-scheme) markdown doc.
- docs: docs/MANUAL-SMOKE-F11.md (live smoke, 10 steps) + README F11 section +
  intro line.

205 unit + 53 host E2E green.

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

docs/MANUAL-SMOKE-F11.md

@@ -0,0 +1,67 @@
+# Manual smoke — F11 preview toolbar as the primary interaction surface (#43)
+
+F11 makes the rendered preview's **header toolbar** the primary interaction
+surface: beside the existing **Annotations** switch it gains a **Pin baseline**
+button and a **single adaptive Ask-Claude button** (Edit Selection when text is
+selected in the preview, Edit Document otherwise). The webview's *visual*
+behavior — the buttons, the live label flip, and the selection→source mapping —
+is verified **here**, not in the automated host E2E (the webview is a sealed
+sandbox; the host's message→seam wiring is covered by the F11 E2E suite). Run
+once per change that touches F11. Live turns hit the SDK; use a trivial
+instruction to keep them quick.
+
+## Setup
+
+1. `npm run build`
+2. Launch the Extension Development Host (F5 in VS Code, or the Run panel) with
+   `sandbox/` open.
+3. Open a markdown document with a few paragraphs (e.g. copy
+   `test/e2e/fixtures/workspace/docs/preview.md`).
+
+## Steps
+
+1. **Right-click gateway (PUC-6).** Right-click the markdown editor **tab** (or
+   use the editor title bar) → **"Cowriting: Open Review Preview"**. The preview
+   opens beside the editor. (The palette entry and `Ctrl+Alt+R` still work too.)
+2. **Three header controls (PUC-1).** The header row shows, left to right: the
+   **☑ Annotations** checkbox (on), a **⌖ Pin baseline** button, and a **✦ Ask
+   Claude to Edit Document** button. They restyle with the theme.
+3. **Adaptive label — selection (PUC-2).** Select a paragraph **in the preview**
+   (drag across rendered text). The Ask-Claude button label flips live to **"✦
+   Ask Claude to Edit Selection"**. Click elsewhere to clear the selection → it
+   flips back to **"✦ Ask Claude to Edit Document"**.
+4. **Edit Selection (PUC-3).** With a paragraph selected in the preview, click
+   **Ask Claude to Edit Selection** → a host input box appears → type an
+   instruction (e.g. "tighten this") → submit. A **blue `cw-proposal` block**
+   appears **inline at that paragraph** with **✓ / ✗** (the selection mapped back
+   to its source block; the edit ran host-side). The editor does **not** change
+   (INV-10). Accept with **✓**: the replacement lands; the block clears.
+5. **Edit Document (PUC-4).** Clear the selection (button reads "Edit Document").
+   Click it → instruct (e.g. "fix any typos and tighten") → submit. Claude's
+   whole-document rewrite surfaces as **one or more** independent blue proposal
+   blocks (one per changed hunk), each independently **✓ / ✗**-able. Accept some,
+   reject others — each behaves as an ordinary F10 proposal.
+6. **Pin baseline (PUC-5).** Make (or accept) a few changes so the preview shows
+   change-marks and a non-zero summary. Click **⌖ Pin baseline**. Expect: the
+   change-marks **clear**, the summary resets to `+0 −0`, and the header epoch
+   reads **`pinned <time>`**. "What changed" now counts from this moment.
+7. **Off-state still maps (INV-36).** Toggle **Annotations** off (clean preview).
+   Selecting a paragraph still flips the button to **Edit Selection** and an edit
+   still works — the off/clean preview is also a selection→source surface.
+8. **Non-authorable doc (PUC-1/7).** Open a markdown doc from a read-only/virtual
+   source (e.g. a Git diff view, or an Output channel rendered as markdown) and
+   open the preview. Expect: **Pin baseline** and **Ask-Claude** render
+   **disabled**; the **Annotations** toggle stays active (reading is allowed).
+9. **Theme.** Toggle light / dark / high-contrast (`Ctrl+K Ctrl+T`). The two
+   toolbar buttons (enabled, hover, disabled states) restyle and stay legible.
+10. **Cleanliness.** `git status` shows nothing written to the document, sidecar,
+    or repo by the toolbar gestures except the edits you explicitly accepted
+    (INV-20/35).
+
+## Pass criteria
+
+All ten steps behave as described; the adaptive label tracks the live selection;
+the selection-scoped edit anchors at the right paragraph; the document edit
+produces per-hunk proposals; pin clears the marks; edit controls are inert on a
+non-authorable doc; no console errors in the webview devtools; nothing is
+persisted by the toolbar beyond the edits you accepted.