docs(f7): manual smoke script + README section (#21)

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

docs/MANUAL-SMOKE-F7.md

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+# Manual smoke — F7 rendered track-changes preview (#21)
+
+The webview's *visual* rendering (mermaid, theming) is verified here, not in the
+automated host E2E (the webview is a sealed sandbox — §6.8). Run once per change
+that touches F7.
+
+## Setup
+
+1. `npm run build`
+2. Launch the Extension Development Host (F5 in VS Code, or the Run panel).
+3. Open a folder and a markdown document containing prose, a `mermaid` fenced
+   diagram, and a `ts` code fence (e.g. copy
+   `test/e2e/fixtures/workspace/docs/preview.md`).
+
+## Steps
+
+1. **Open the preview.** Run **"Cowriting: Open Track-Changes Preview"** (or
+   `Ctrl+Alt+R`). A preview opens beside the editor, rendering the document.
+   Header reads `Track changes since opened …`, summary `+0 −0`. The mermaid
+   diagram renders as a diagram; the code fence renders as highlighted code.
+2. **Edit prose.** In the source editor, change a word in a paragraph. The
+   preview updates (≈150 ms): the old word struck (`<del>`), the new word
+   highlighted (`<ins>`); the summary increments.
+3. **Edit the mermaid.** Change `a --> b` to `a --> c`. The preview re-renders the
+   **new** diagram with a **"changed"** badge at its top-right.
+4. **Ask Claude + accept.** Select the target sentence → "Ask Claude to Edit
+   Selection" → accept the proposal. The accepted text **drops its marks** (the
+   baseline advanced; PUC-3).
+5. **Pin.** Run "Cowriting: Pin Diff Baseline to Now". All marks clear (baseline
+   == now; PUC-4).
+6. **Theme.** Toggle light/dark (`Ctrl+K Ctrl+T`). Marks and the diagram restyle
+   to the theme.
+7. **Cleanliness.** `git status` shows nothing changed by the preview (INV-20).
+
+## Pass criteria
+
+All seven steps behave as described; no console errors in the webview devtools;
+nothing written to the document, sidecar, or repo.