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

crates/ocs_plugin_api/src/ipc/v4/client.rs

@@ -761,7 +761,6 @@ impl DocumentReader for EmptyDocumentReader {
 #[cfg(all(test, feature = "host"))]
 mod tests {
     use std::sync::atomic::{AtomicU64, Ordering};
-    use std::sync::Mutex as StdMutex;
     use std::thread;
 
     use interprocess::local_socket::{
@@ -772,10 +771,9 @@ mod tests {
     use super::*;
     use crate::ipc::transport::recv;
     use crate::ipc::v4::protocol::{HostToPluginV4, PluginToHostV4};
+    use crate::test_lock::ENV_LOCK;
     use acadrust::entities::Point;
 
-    static ENV_LOCK: StdMutex<()> = StdMutex::new(());
-
     fn unique_socket_name() -> String {
         static COUNTER: AtomicU64 = AtomicU64::new(0);
         let n = COUNTER.fetch_add(1, Ordering::Relaxed);