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/runner.rs

@@ -4,6 +4,18 @@
 //! (`--ocs-plugin-runner <socket> <cdylib>`). Keeping the runner code inside
 //! `ocs_plugin_api` means the host only needs to know the CLI contract, not the
 //! internal plugin-loading and IPC details.
+//!
+//! Entry points and loops:
+//!
+//! - [`run`] — loads the cdylib with `libloading`, connects back to the host,
+//!   and dispatches to either the V2/V3 loop or the V4 loop based on the plugin's
+//!   declared API version.
+//! - `run_v3` — synchronous request/response loop over [`crate::ipc::client::IpcClient`].
+//! - `run_v4` — multiplexed frame loop over [`crate::ipc::v4::client::V4Client`];
+//!   drains notifications and dispatches host requests.
+//!
+//! Panics inside plugin callbacks are caught by `catch_unwind` and converted to
+//! error responses so a buggy plugin callback does not tear down the runner.
 
 use std::cell::RefCell;
 use std::collections::HashMap;