docs(f6): manual smoke + README pointer (SLICE-4)

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

README.md

@@ -13,7 +13,8 @@ catalog (a pure, key-free SDK call) in a notification and the
 
 Features shipped so far: F2 region-anchored threads (Feature #4), F3 live
 human/Claude attribution (Feature #6), F4 propose/accept diff flow
-(Feature #12).
+(Feature #12), F5 cross-rung sidecar contract (Feature #14), F6 diff-view
+toggle (Feature #17).
 
 ## Architecture
 
@@ -131,6 +132,32 @@ record per the contract; git push/pull is the transport, no re-homing ever.
 
 Design: `vscode-cowriting-plugin-content/specs/coauthoring-cross-rung-format.md`.
 
+## F6 — Diff-view toggle (Feature #17)
+
+**Cmd/Ctrl-Alt-D** (or **Cowriting: Toggle Diff View**) flips a tracked
+document into a native `vscode.diff` against a **coauthoring baseline** — the
+readonly baseline on the left, your **live, editable** document on the right
+(so you keep writing inside the diff; toggling again closes it). The diff
+answers "what did *I* change?" in one keystroke instead of git archaeology.
+
+- **Machine-factored baseline (INV-18):** the baseline initializes when a doc
+  is first tracked and **advances automatically at every machine landing**
+  (every successful `applyAgentEdit` seam apply — INV-9). So text Claude landed
+  never shows as a change; everything the diff shows is operator-authored by
+  construction — no attribution filtering.
+- **Pin on demand:** **Cowriting: Pin Diff Baseline to Now** resets the
+  baseline to the current buffer for a deliberate "review my next pass" epoch;
+  the diff tab title names the epoch (`opened` / `Claude landed` / `pinned`).
+- **Pure view, repo-free (INV-19):** the baseline snapshot lives in VS Code
+  workspace storage, **never** the repo — `.threads/`, the cross-rung contract
+  (INV-14..17), and `SCHEMA_VERSION` are untouched. Storage-unavailable
+  degrades to in-memory baselines + one warning.
+- **No LLM in CI:** host E2E (`test/e2e/suite/diffView.test.ts`) drives the same
+  programmatic seam ingress (propose + accept) the F4 suite uses.
+
+Design: `vscode-cowriting-plugin-content/specs/coauthoring-diff-view.md`.
+Live smoke: [`docs/MANUAL-SMOKE-F6.md`](docs/MANUAL-SMOKE-F6.md).
+
 ## Develop
 
 - `npm run watch` — rebuild on change.

docs/MANUAL-SMOKE-F6.md

@@ -0,0 +1,26 @@
+# Manual smoke — F6 diff-view toggle (live)
+
+Pre-req: a real machine (not CI). Step 4 uses a live Claude turn — Claude Code
+installed + signed in (the `claude-code` provider rides that login, INV-8). The
+rest of F6 needs no credentials and no network.
+
+1. `npm run build`, launch the extension (F5 in VS Code opens the committed
+   `sandbox/` playground), open `playground.md`.
+2. Edit a sentence by hand, then **Cmd/Ctrl-Alt-D** (or run **Cowriting: Toggle
+   Diff View**). ✅ A diff opens: the readonly baseline on the left, your live
+   document on the right; the tab title reads `playground.md — my changes since
+   opened <time>`. Your hand edit shows as a change.
+3. Keep typing **in the diff's right pane**. ✅ Editing continues in place;
+   decorations/threads still work. Toggle again. ✅ The diff tab closes and the
+   normal editor is back.
+4. Select a sentence → right-click → **Ask Claude to Edit Selection** → give an
+   instruction → **✓ Accept Proposal**. Now toggle the diff. ✅ The accepted
+   text is **not** shown as a change (the baseline advanced past the landing —
+   INV-18); only your own subsequent edits show.
+5. Run **Cowriting: Pin Diff Baseline to Now**. ✅ The diff empties (baseline =
+   now); the title reads `pinned <time>`. Type → your changes show against the
+   pin.
+6. Reload the window (Developer: Reload Window). ✅ Toggle: the baseline is the
+   same as before the reload (persisted in workspace storage, not the repo —
+   INV-19; `git status` shows nothing new).
+7. Toggle on a file outside the workspace folder. ✅ A warning, no diff.