feat(plugin): add layer creation and modification host API

- Add LayerConfig struct and add_layer / modify_layer methods to HostApi trait
- Implement IPC request/response serialization for AddLayer and ModifyLayer
- Implement HostSession methods with CAD property defaults and selective modification
- Refresh layer panel UI and invalidate scene layer dependencies upon layer changes
- Add unit tests for add_layer with defaults and partial modify_layer

crates/ocs_plugin_api/src/ipc/protocol.rs

@@ -138,6 +138,10 @@ pub enum PluginRequest {
     GetTabId,
     /// V5: ask the host for the filesystem path of the document in `tab_id`.
     DocumentPath { tab_id: u64 },
+    /// Add a layer to the active document with full initial properties.
+    AddLayer(crate::host::LayerConfig),
+    /// Modify specified properties of an existing layer in the active document.
+    ModifyLayer(crate::host::LayerConfig),
 }
 
 /// Responses the host sends back for `PluginRequest`.
@@ -164,6 +168,8 @@ pub enum PluginResponse {
     TabId(u64),
     /// V5: filesystem path of the document in the requested tab, if any.
     DocumentPath(Option<std::ffi::OsString>),
+    /// Optional entity handle (e.g. from AddLayer).
+    OptHandle(Option<Handle>),
 }
 
 /// Messages sent from the host to the plugin runner.