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

rust/crates/mnote-web/src/ssr/pages/layout.rs

@@ -6745,6 +6745,36 @@ const SIDEBAR_TREE_JS: &str = r##"
   }
   scheduleInitializePageUiSurfaces();
 
+  function installMnoteDevHotReload() {
+    var bootId = '';
+    var failedOnce = false;
+    var timer = 0;
+    function tick() {
+      fetch('/api/dev/hot-reload', { cache: 'no-store', headers: { accept: 'application/json' } })
+        .then(function(response) { return response.ok ? response.json() : null; })
+        .then(function(payload) {
+          if (!payload || payload.enabled !== true || !payload.bootId) {
+            if (!bootId && timer) window.clearInterval(timer);
+            return;
+          }
+          document.documentElement.setAttribute('data-mnote-dev-hot-reload', 'enabled');
+          if (!bootId) {
+            bootId = String(payload.bootId);
+            return;
+          }
+          if (failedOnce || String(payload.bootId) !== bootId) {
+            window.location.assign(window.location.href);
+          }
+        })
+        .catch(function() {
+          if (bootId) failedOnce = true;
+        });
+    }
+    tick();
+    timer = window.setInterval(tick, 1000);
+  }
+  installMnoteDevHotReload();
+
   function readPageDragNodeId(event) {
     var fromTransfer = event.dataTransfer ? event.dataTransfer.getData(PAGE_DRAG_MIME) || event.dataTransfer.getData('text/plain') : '';
     return (fromTransfer || draggingPageNodeId || '').trim();
@@ -7437,6 +7467,14 @@ mod tests {
         assert!(!SIDEBAR_TREE_JS.contains("window.location.reload"));
     }
 
+    #[test]
+    fn sidebar_tree_runtime_contains_dev_hot_reload_client() {
+        assert!(SIDEBAR_TREE_JS.contains("installMnoteDevHotReload"));
+        assert!(SIDEBAR_TREE_JS.contains("/api/dev/hot-reload"));
+        assert!(SIDEBAR_TREE_JS.contains("data-mnote-dev-hot-reload"));
+        assert!(SIDEBAR_TREE_JS.contains("window.clearInterval(timer)"));
+    }
+
     #[test]
     fn sidebar_tree_runtime_renders_context_menu_and_scoped_title_updates() {
         assert!(SIDEBAR_TREE_JS.contains("openTreeContextMenu"));