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/mod.rs

@@ -4,6 +4,15 @@
 //! requests, responses, and best-effort notifications over a single local
 //! socket. The V3 files (`src/ipc/{protocol,client,server}.rs`) are left
 //! untouched; all V4 logic lives in this module.
+//!
+//! Submodules:
+//!
+//! - [`protocol`](crate::ipc::v4::protocol) — V4 wire frames, notification
+//!   envelopes, and correlation ids.
+//! - [`client`](crate::ipc::v4::client) — plugin-side V4 client loop and
+//!   notification dispatch.
+//! - [`server`](crate::ipc::v4::server) — host-side V4 connection and reader
+//!   thread.
 
 pub mod client;
 pub mod protocol;