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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>