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.