chore: document architecture gaps and add dev hot reload

- add npm dev:hot wrapper using cargo-watch and page reload polling

- add mnote-web dev hot reload endpoint and coverage

- record current architecture review and tracked bug findings across realtime, tree, editor, and AI runtimes

Verification:

- node scripts/task-dev-hot-plan-test.js

- node --check scripts/dev-hot.js

- cargo test -p mnote-web dev_hot -- --nocapture

package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "desktop": "node scripts/desktop-prod.js",
     "desktop:hot": "node scripts/desktop-hot.js",
+    "dev:hot": "node scripts/dev-hot.js",
     "desktop:local": "node scripts/desktop-local.js",
     "desktop:electron": "node desktop-electron/dev-runner.js",
     "build:desktop:next": "node scripts/prepare-desktop-next.js",