feat(plugin): emit SelectionChangedV4 to V4 plugins and add ocs_plugin_api architecture docs

- Add HostNotification::SelectionChangedV4 { tab_id, handles } with discriminant 7
- Broadcast active-tab selection changes at update() and automation_op boundaries
- Use stable order-independent signature to avoid spurious hover notifications
- Clear Scene selection in automation new/open to avoid stale handles
- Add crates/ocs_plugin_api/ARCHITECTURE.md and module-level rustdoc
- Cross-link README.md and docs/plugin-architecture.md
- Fix test env race with shared ENV_LOCK and EnvVarGuard helpers

src/plugin/v4_support.rs

@@ -42,6 +42,15 @@ pub fn on_tab_closed(tab_id: u64) {
     broadcast(HostNotification::DocumentTabClosed { tab_id });
 }
 
+/// Broadcast a selection change to V4 plugins.
+///
+/// `HostNotification::SelectionChangedV4` carries the active tab id and the
+/// current selection set. The caller is responsible for only calling this when
+/// the selection actually changed; see `OpenCADStudio::notify_plugins_selection_changed`.
+pub fn publish_selection_changed_v4(tab_id: u64, handles: Vec<acadrust::Handle>) {
+    broadcast(HostNotification::SelectionChangedV4 { tab_id, handles });
+}
+
 /// V4 notification handler installed on the plugin manager.
 ///
 /// Forwards REPL status messages to the log; other notifications are handled