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/server.rs

@@ -98,6 +98,8 @@ 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)),
         GetSystemVariable { name } => PluginResponse::SystemVariable(host.system_variable(&name)),
         SetSystemVariable { name, value } => {
             PluginResponse::SystemVariableResult(host.set_system_variable(&name, value))