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/host.rs

@@ -552,6 +552,37 @@ pub trait HostApi {
         let _ = tab_id;
         None
     }
+
+    /// Add a layer to the active document with full initial properties.
+    /// If an optional property in `config` is `None`, standard CAD defaults are applied.
+    /// Returns `None` if the layer already exists or `config.name` is invalid.
+    fn add_layer(&mut self, config: LayerConfig) -> Option<Handle> {
+        let _ = config;
+        None
+    }
+
+    /// Modify specified properties of an existing layer in the active document.
+    /// Properties that are `None` in `config` are left untouched as-is.
+    /// Returns `false` if the layer does not exist or `config.name` is invalid.
+    fn modify_layer(&mut self, config: LayerConfig) -> bool {
+        let _ = config;
+        false
+    }
+}
+
+/// Configuration properties for creating or modifying a layer.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Default)]
+pub struct LayerConfig {
+    pub name: String,
+    pub color: Option<acadrust::types::Color>,
+    pub linetype: Option<String>,
+    pub lineweight: Option<acadrust::types::LineWeight>,
+    pub off: Option<bool>,
+    pub frozen: Option<bool>,
+    pub locked: Option<bool>,
+    pub plottable: Option<bool>,
+    pub transparency: Option<acadrust::types::Transparency>,
+    pub description: Option<String>,
 }
 
 /// Simplified, read-only entity kind exposed by [`DocumentReader`].

crates/ocs_plugin_api/src/ipc/client.rs

@@ -416,6 +416,34 @@ impl HostApi for PluginHostApi {
             }
         }
     }
+
+    fn add_layer(&mut self, config: crate::host::LayerConfig) -> Option<Handle> {
+        match self.client.request(PluginRequest::AddLayer(config)) {
+            Ok(PluginResponse::OptHandle(h)) => h,
+            Ok(other) => {
+                eprintln!("[plugin] unexpected AddLayer response: {other:?}");
+                None
+            }
+            Err(e) => {
+                eprintln!("[plugin] AddLayer request failed: {e}");
+                None
+            }
+        }
+    }
+
+    fn modify_layer(&mut self, config: crate::host::LayerConfig) -> bool {
+        match self.client.request(PluginRequest::ModifyLayer(config)) {
+            Ok(PluginResponse::Bool(b)) => b,
+            Ok(other) => {
+                eprintln!("[plugin] unexpected ModifyLayer response: {other:?}");
+                false
+            }
+            Err(e) => {
+                eprintln!("[plugin] ModifyLayer request failed: {e}");
+                false
+            }
+        }
+    }
 }
 
 /// Sentinel reader used when the shared-memory view could not be initialized.

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.

crates/ocs_plugin_api/src/ipc/server.rs

@@ -77,5 +77,7 @@ pub fn handle_plugin_request(
         DocumentPath { tab_id } => PluginResponse::DocumentPath(
             host.document_path(tab_id).map(|path| path.into_os_string()),
         ),
+        AddLayer(config) => PluginResponse::OptHandle(host.add_layer(config)),
+        ModifyLayer(config) => PluginResponse::Bool(host.modify_layer(config)),
     }
 }

crates/ocs_plugin_api/src/ipc/v4/client.rs

@@ -730,6 +730,34 @@ impl HostApi for V4PluginHostApi {
             }
         }
     }
+
+    fn add_layer(&mut self, config: crate::host::LayerConfig) -> Option<Handle> {
+        match self.request(PluginRequest::AddLayer(config)) {
+            Ok(PluginResponse::OptHandle(h)) => h,
+            Ok(other) => {
+                eprintln!("[plugin] unexpected AddLayer response: {other:?}");
+                None
+            }
+            Err(e) => {
+                eprintln!("[plugin] AddLayer request failed: {e}");
+                None
+            }
+        }
+    }
+
+    fn modify_layer(&mut self, config: crate::host::LayerConfig) -> bool {
+        match self.request(PluginRequest::ModifyLayer(config)) {
+            Ok(PluginResponse::Bool(b)) => b,
+            Ok(other) => {
+                eprintln!("[plugin] unexpected ModifyLayer response: {other:?}");
+                false
+            }
+            Err(e) => {
+                eprintln!("[plugin] ModifyLayer request failed: {e}");
+                false
+            }
+        }
+    }
 }
 
 #[cfg(test)]