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/transport.rs

@@ -1,4 +1,12 @@
 //! Length-framed transport over `interprocess::local_socket` streams.
+//!
+//! Messages are serialized with `bincode`, prefixed by a little-endian `u64`
+//! length, and sent over the stream. The receiver parses the length, bounds it
+//! against [`MAX_MESSAGE_SIZE`], then deserializes the payload.
+//!
+//! [`send`] and [`recv`] are synchronous and block until the full frame is read
+//! or the peer disconnects. `Disconnected` is returned on clean EOF; `TooLarge`
+//! rejects messages that exceed 64 MiB to protect host/runner memory.
 
 use std::io::{Read, Write};