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/host_v4.rs

@@ -3,6 +3,12 @@
 //! Maintains a per-tab shared-memory document view keyed by the stable
 //! `DocumentTab.id`. The manager is used by `src/plugin/v4_support.rs` to
 //! open, publish, and close V4 snapshots independently of the V3 reader path.
+//!
+//! [`manager`] returns a global `Mutex`-guarded [`HostV4SnapshotManager`].
+//! [`HostV4SnapshotManager::open`] lazily creates a [`DocumentSnapshotStore`] for
+//! a tab; [`HostV4SnapshotManager::publish`] updates an existing store;
+//! [`HostV4SnapshotManager::close`] drops it. Segment size defaults to 16 MiB
+//! and can be overridden with `OCS_V4_SNAPSHOT_SEGMENT_SIZE`.
 
 use std::collections::HashMap;
 use std::sync::{Mutex, OnceLock};