feat(plugin-api): entity-class display declarations on the manifest (API v10)

Plugins can now declare the entity classes they identify via XDATA, and the
host resolves them to display-only type labels while the declaring plugin is
loaded.

- Append `BuiltinPlugin::entity_classes()` (a guarded trailing vtable slot,
  the same additive pattern as `on_load` in v5) returning a &'static
  `EntityClasses { app_id, class_prefix, labels }`. The static
  `PluginManifest` struct is deliberately untouched: older cdylibs share it
  by pointer and its layout is frozen ABI since v1, so adding a field would
  read past their allocation.
- `OwnedEntityClasses` travels on the registered manifest as
  `OwnedPluginManifest.entity_classes` with `#[serde(default)]`, so manifests
  serialized before the field keep decoding (older runners tolerate it
  both ways).
- The runner folds the declaration into the GetManifest registration only
  when the loaded cdylib's exported `ocs_plugin_api_version` symbol reports
  >= 10 — an older binary's vtable predates the slot, so it is never called;
  its entities simply keep their native type names.
- `PluginManager::entity_class_declarations` exposes the live set, filtered
  to alive, non-disabled plugins in load order. The registered manifests are
  the only source and nothing is cached: not installed, disabled,
  cap-excluded (`OCS_PLUGIN_MAX_API_VERSION`), failed at load, removed, or a
  dead process all contribute nothing.
- Embedded version info `api_version` follows the bump to 10;
  `API_VERSION_MIN_SUPPORTED` stays 3.

Tests: runner fold (v10 folds, pre-v10 skipped, `entity_classes()` panic is
contained and reported), manager filtering (disabled / dead / undeclaring
skipped; declarations disappear once the process dies — verified with a real
killed child), bincode registration round-trip, and older-manifest serde
default.

crates/ocs_plugin_api/src/ribbon/owned.rs

@@ -123,6 +123,18 @@ fn collect_item_command_ids(item: &OwnedRibbonItem, out: &mut Vec<String>) {
     }
 }
 
+/// Owned mirror of [`crate::manifest::EntityClasses`]: the entity-class
+/// declaration as it travels on the registered manifest across the IPC wire.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct OwnedEntityClasses {
+    /// XDATA application name whose records carry the class-identity keys.
+    pub app_id: String,
+    /// Prefix that marks a string XDATA value as a class-identity key.
+    pub class_prefix: String,
+    /// Class key → display label.
+    pub labels: std::collections::BTreeMap<String, String>,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct OwnedPluginManifest {
     pub id: String,
@@ -133,6 +145,11 @@ pub struct OwnedPluginManifest {
     pub ribbon_order: i32,
     pub xdata_apps: Vec<String>,
     pub command_prefixes: Vec<String>,
+    /// Entity-class display declarations (API v10), folded in by the runner
+    /// for plugins that declare them. `#[serde(default)]` keeps manifests
+    /// serialized before the field existed decoding as `None`.
+    #[serde(default)]
+    pub entity_classes: Option<OwnedEntityClasses>,
 }
 
 impl From<IconKind> for OwnedIconKind {