feat(plugin): ModuleEvent::PluginFileDialog for native file import

Add-ons had no way to request a file picker — only the host's own Open
dialog. PluginFileDialog lets a plugin tool ask the host to open a native
picker; on selection the host dispatches "<command> <path>" back to the
plugin with original case preserved (bypassing the command-line
upper-casing that mangles case-sensitive paths on Linux/macOS). The demo
plugin gains an Import tool exercising it.

Part of the #100 extensibility epic (surfaced in #106).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/plugin/registry.rs

@@ -139,6 +139,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn plugin_file_dialog_dispatch_preserves_case() {
+        // ModuleEvent::PluginFileDialog dispatches "<command> <path>" verbatim;
+        // the mixed-case path must reach the plugin unaltered.
+        let mut app = OpenCADStudio::new_for_test();
+        let line = "DP_IMPORT /home/User/My Points.CSV";
+        assert!(try_dispatch(&mut app, 0, line));
+        let info = app.command_history_info();
+        assert!(
+            info.iter().any(|t| t.contains("/home/User/My Points.CSV")),
+            "path case not preserved; info: {info:?}"
+        );
+    }
+
     #[test]
     fn unknown_plugin_command_falls_through() {
         let mut app = OpenCADStudio::new_for_test();