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/host.rs

@@ -16,7 +16,7 @@ use std::path::PathBuf;
 
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
-use crate::manifest::PluginManifest;
+use crate::manifest::{EntityClasses, PluginManifest};
 use crate::ribbon::CadModule;
 
 // Re-export the opencadcodec crate and the types that appear in the HostApi trait
@@ -659,6 +659,21 @@ pub trait BuiltinPlugin: Send + Sync {
     /// Added in API v5. The runner only calls this for plugins that report API
     /// major 5 or newer.
     fn on_load(&mut self, _host: &mut dyn HostApi) {}
+
+    /// Entity-class display declarations (API v10): the XDATA
+    /// class-identity keys the plugin stamps on entities, mapped to the
+    /// display labels the host shows while this plugin is loaded (e.g. the
+    /// Properties panel title becomes `"Linear Element (Line)"`). Display-only
+    /// — the labels never round-trip the drawing and never govern behavior;
+    /// entities carry only the stable identity keys in their XDATA.
+    ///
+    /// The default (`None`) declares no entity classes. The runner only calls
+    /// this for plugins whose exported `ocs_plugin_api_version` symbol reports
+    /// API major 10 or newer, so older cdylibs are never touched past their
+    /// vtable.
+    fn entity_classes(&self) -> Option<&'static EntityClasses> {
+        None
+    }
 }
 
 /// A point-driven interactive command a plugin starts via