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/process/manager.rs

@@ -1,4 +1,19 @@
 //! Process manager for out-of-process plugins.
+//!
+//! `PluginManager` is the host's owner of every loaded plugin process. It is
+//! responsible for:
+//!
+//! - Spawning plugins via [`PluginProcess::spawn`] and building their ribbon
+//!   modules.
+//! - Routing commands to plugins through [`PluginManager::dispatch`].
+//! - Broadcasting host notifications to every alive V4 plugin via
+//!   [`PluginManager::broadcast_notification`].
+//! - Surfacing dead plugins and per-plugin errors in [`DispatchResult`].
+//!
+//! Notifications are V4-only: `broadcast_notification` skips non-V4 processes.
+//! The plugin-to-host notification handler (set via
+//! [`PluginManager::set_notification_handler`]) runs on the V4 reader thread
+//! and must not block. For heavy work, forward the notification to a channel.
 
 use std::path::Path;
 use std::sync::Arc;