fix(plugins): gate on_load behind API v5

BREAKING CHANGE: plugins using on_load must target API v5 and declare api_version = 5.

Refs #901

crates/ocs_plugin_api/README.md

@@ -9,7 +9,7 @@ cheaply. The runtime host surface is enabled by the `host` feature.
 ## Plugin Architecture
 
 OCS is scriptable and extensible through a versioned plugin API. The API
-supports three major protocol generations: **V2**, **V3**, and **V4**. Each
+supports four major generations: **V2**, **V3**, **V4**, and **V5**. Each
 generation keeps the previous ABI stable by appending new vtable entries and new
 enum variants at the end, so older plugins continue to load on newer hosts.
 
@@ -18,12 +18,17 @@ enum variants at the end, so older plugins continue to load on newer hosts.
 | **V2** | Out-of-process runner | Synchronous commands + interactive point collection (`HostApi::start_interactive`) | Baseline |
 | **V3** | Out-of-process runner | Local cached `document()`/`document_mut()` + shared-memory `DocumentReader`/`document_view` for large reads | New vtable entries appended; new `PluginRequest`/`PluginResponse` variants appended |
 | **V4** | Out-of-process runner over a multiplexed local socket | Full-duplex notifications + asynchronous REPL `ExecuteCode` | New `HostToPluginV4`/`PluginToHostV4` frame layer; `BuiltinPlugin::on_notification` and `start_execute_code` appended to trait |
+| **V5** | V4 multiplexed runner | One-shot `BuiltinPlugin::on_load` initialization | Lifecycle callback appended to `BuiltinPlugin` |
 
 The host controls which generations are accepted at runtime through
-`OCS_PLUGIN_MAX_API_VERSION` (e.g. `2` for V2-only mode, `3` to disable V4). The
-current host advertises [`API_VERSION = 4`](src/manifest.rs) and supports plugins
+`OCS_PLUGIN_MAX_API_VERSION` (e.g. `2` for V2-only mode, `4` to disable V5). The
+current host advertises [`API_VERSION = 5`](src/manifest.rs) and supports plugins
 back to `API_VERSION_MIN_SUPPORTED = 2`.
 
+V5 calls `BuiltinPlugin::on_load` once after the runner connects and before the
+first user command. Plugins can cache `HostApi::plugin_request_sender()` there
+for worker-thread access. A panic in the callback fails plugin loading.
+
 ### High-level layout
 
 ```mermaid
@@ -263,7 +268,7 @@ static MANIFEST: PluginManifest = PluginManifest {
     name: "Hello Plugin",
     version: "0.1.0",
     description: "A minimal example plugin.",
-    api_version: ApiVersion { major: 4 },
+    api_version: ApiVersion::CURRENT,
     ribbon_order: 100,
     xdata_apps: &[],
     command_prefixes: &["HELLO"],