Merge origin/main into plugin/host-model-api

Conflicts were additive: main's AddLayer/ModifyLayer (LayerConfig, OptHandle)
alongside this branch's v7 system-variable, transaction, selection and solid
operations. Main's variants keep their positions in the IPC enums; the v7
additions follow them. src/io/mod.rs takes main's reworded Shape-rotation note.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/ocs_plugin_api/src/ipc/client.rs

@@ -490,6 +490,34 @@ impl HostApi for PluginHostApi {
             Err(error) => Err(error.to_string()),
         }
     }
+
+    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.