Add design system feature flag and runtime integration

docs/deployment-notes.md

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+# Deployment Notes
+
+## FEATURE_DESIGN_SYSTEM rollout
+
+- **Default:** `false` (legacy styling). The flag lives in `js/constants.js` and is exposed through `window.__BITVID_RUNTIME_FLAGS__` at runtime.
+- **Enable for canaries:** Before deploying a canary, run `window.__BITVID_RUNTIME_FLAGS__.FEATURE_DESIGN_SYSTEM = true` (or call `setFeatureDesignSystemEnabled(true)`) in the browser console. The change propagates immediately, flipping every `[data-ds]` container to `data-ds="new"` so controllers can hydrate the updated primitives.
+- **Disable / rollback:** Set the flag back to `false` via the runtime object or redeploy with the default configuration. All root containers revert to `data-ds="legacy"`, forcing controllers and templates to stay on the existing class list.
+- **Template contract:** Every layout, view, and partial now includes `data-ds` on the root element. Keep this attribute intact when editing HTML so the runtime can swap modes without re-rendering.
+- **Controllers:** UI controllers receive a `designSystem` context in their constructors. Call `context.isNew()` before attaching new components or classes, and lean on `context.getMode()` when logging or telemetry need to differentiate between legacy and new renders.