feat(plugins): expose document path to plugins via HostApi

OpenCADStudio/plugins#901 (comment)

- Add HostApi::document_path(tab_id) returning Option<PathBuf>.
- Add PluginRequest::DocumentPath and PluginResponse::DocumentPath for
  out-of-process plugins.
- Implement the request in the IPC server and in both V3/V4 plugin-side
  HostApi proxies.
- Implement HostSession::document_path by looking up the tab by id and
  returning its current_path.

crates/ocs_plugin_api/src/host.rs

@@ -12,6 +12,7 @@
 //! [`ensure_plugin_state`] helpers for the ergonomic typed access.
 
 use std::any::Any;
+use std::path::PathBuf;
 
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
@@ -531,6 +532,15 @@ pub trait HostApi {
     fn add_entities(&mut self, entities: Vec<EntityType>) -> Vec<Handle> {
         entities.into_iter().map(|e| self.add_entity(e)).collect()
     }
+
+    /// Return the filesystem path of the document in tab `tab_id`, if any.
+    /// Added after API v4; appended at the very end so older plugins compiled
+    /// without it keep stable vtable indices. The default returns `None`;
+    /// in-process hosts should override it to expose the real path.
+    fn document_path(&self, tab_id: u64) -> Option<PathBuf> {
+        let _ = tab_id;
+        None
+    }
 }
 
 /// Simplified, read-only entity kind exposed by [`DocumentReader`].