docs(hook-manager): document demo persistence and rollback toast; add browser test steps

Author: SorraTheOrc

src/runtime/hook-manager/README.md

@@ -23,3 +23,37 @@ hm.on('audit', persistence.audit);
 await hm.emitParallel('pre_inject', { branchId: 'b1' });
 
 Note: subscribers must be resilient; HookManager will catch and surface handler results without throwing.
+
+Demo additions in this branch:
+
+- `src/runtime/subscribers/demo-persistence.js` — writes debug `.save` artifacts to `src/.saves/` on `post_checkpoint` and `on_rollback` (dev/demo only).
+- `web/demo/js/runtime-hooks-shim.js` — attempts to register the demo persistence subscriber when loaded in a bundler/node-like environment so debug saves are produced during local dev and CI.
+- `web/demo/js/inkrunner.js` — shows a lightweight rollback toast UI when a load fails. The toast is demo-only and displays a pointer to `src/.saves/` where debug artifacts are written.
+
+How to test the rollback toast in a browser:
+
+1) Start the demo server:
+   - `npm run serve-demo -- --port 4173`
+   - Open `http://127.0.0.1:4173/demo` in your browser.
+
+2) Cause a load failure to trigger rollback handling. Two quick options:
+   - Corrupt the saved payload:
+     a) Open DevTools → Console
+     b) Run: `localStorage.setItem('ge-hch.smoke.save', 'not-a-json')`
+     c) Click the "Load" button in the demo page or run: `window.__inkrunner.loadState()`
+     d) You should see a red toast at the bottom-right with a message referencing `/src/.saves/`.
+
+   - Remove InkJS or simulate an error during load:
+     a) In DevTools Console run: `delete window.inkjs` then click "Load" or call `window.__inkrunner.loadState()`.
+     b) The toast should appear when the load fails and `on_rollback` is emitted.
+
+3) Inspect debug saves:
+   - On successful `post_checkpoint` events the demo persistence subscriber writes files to `src/.saves/`.
+   - On failures the subscriber writes a small rollback debug save you can inspect.
+   - Inspect saved files from your shell: `ls -l src/.saves && cat src/.saves/<file>.save`.
+
+Notes & caveats:
+- The toast and demo persistence are intentionally lightweight and for development only; they rely on the bundler/node environment to register persistence. Plain static-only page loads may not register the demo persistence subscriber.
+- The debug save directory is `src/.saves/` in the repository root. CI artifacts may be stored elsewhere depending on CI config.
+
+If you want, I can also add a query-parameter toggle (e.g., `?demo_persistence=1`) so the demo registers the subscriber even on static loads. Would you like that?