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/src/manifest.rs

@@ -17,8 +17,9 @@
 /// `document()` / `document_mut()` to local cached copies for out-of-process
 /// plugins and appends `document_reader` / `document_view` at the end of the
 /// vtable so API v2 plugins keep working. v4 adds full-duplex notifications on
-/// a multiplexed socket while leaving the V2/V3 ABI and protocol untouched.
-pub const API_VERSION: u32 = 4;
+/// a multiplexed socket while leaving the V2/V3 ABI and protocol untouched. v5
+/// adds the `BuiltinPlugin::on_load` lifecycle callback.
+pub const API_VERSION: u32 = 5;
 
 /// Oldest plugin API major the current host still loads. This keeps previously
 /// compiled cdylibs usable as long as their vtable layout is a prefix of the
@@ -26,9 +27,8 @@ pub const API_VERSION: u32 = 4;
 pub const API_VERSION_MIN_SUPPORTED: u32 = 2;
 
 /// Environment variable that caps the API major accepted by the host at
-/// runtime. Set to `3` to disable V4 plugins while keeping V2/V3 plugins
-/// working; set to `2` for V2-only mode. The same mechanism will apply to
-/// future API majors.
+/// runtime. Set to `4` to disable V5 plugins, `3` to disable V4, or `2` for
+/// V2-only mode.
 pub const MAX_API_VERSION_ENV: &str = "OCS_PLUGIN_MAX_API_VERSION";
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]