docs(f9): manual smoke + README authorship-mode note

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/MANUAL-SMOKE-F9.md

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+# Manual smoke — F9 authorship view in the preview
+
+Confirms the rendered preview's Authorship mode colors Claude's vs your text. One
+live turn hits the SDK.
+
+## Prereqs
+- `npm run build`; launch the Extension Development Host (F5) with `sandbox/` open.
+
+## Steps
+1. Open a markdown file. Open **Cowriting: Open Track-Changes Preview** (`Ctrl+Alt+R`).
+2. The header shows a `[ Track changes | Authorship ]` toggle. It opens in **Track
+   changes** (unchanged behavior).
+3. Select a sentence → **Ask Claude to Edit Selection** → instruct → accept.
+4. Click **Authorship**. Expect: Claude's accepted text is tinted (blue) and your
+   own typing is tinted (green); the header shows a legend (Claude / You). Text you
+   never touched (original content) is plain.
+5. Type a few words yourself, mixing into a Claude sentence. Expect: the colors
+   split mid-paragraph at the exact boundaries.
+6. If the doc has a mermaid or code fence Claude authored, expect the whole fence
+   to carry a small author badge (not per-character).
+7. Flip back to **Track changes**. Expect: the diff view is exactly as before.