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/protocol.rs

@@ -4,6 +4,16 @@
 //! (expecting a response) or a response (to a previous request). This lets the
 //! host handle plugin RPCs inline while it waits for the result of a host→plugin
 //! request such as `Dispatch`, avoiding the need for two sockets or threads.
+//!
+//! Compatibility rule: new variants are appended at the end of every public enum
+//! (`HostRequest`, `HostResponse`, `PluginRequest`, `PluginResponse`,
+//! `RunnerHandshake`) so older plugins keep their bincode discriminant indices.
+//! The V4 additions are a separate frame layer in [`crate::ipc::v4`] and do not
+//! alter these enums.
+//!
+//! The pre-shared runner token is delivered through [`PLUGIN_TOKEN_ENV`]
+//! (`OCS_PLUGIN_TOKEN`). The runner must present the same token immediately
+//! after connecting or the host closes the connection.
 
 use serde::{Deserialize, Serialize};