feat(automation): headless JSON server (--serve) + Python client

Add an external automation API (issue #29 / #100 track 2): `OpenCADStudio
--serve` runs without a GUI and is driven over a line-based JSON protocol
on stdin/stdout — open / new / run / entities / save. State persists
across requests so a script or AI agent can act, observe, and act again.

`run` drives the app's existing command system rather than a separate
binding, so coverage grows with the app; synchronous commands apply now,
pick-based interactive ones come once coordinate feeding is wired. Ships
a ~100-line example ocs.py client over the same protocol — no FFI to
maintain. Headless works because the app already constructs and dispatches
GUI-less in tests.

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

src/main.rs

@@ -39,6 +39,13 @@ fn main() -> iced::Result {
     }
     #[cfg(not(target_arch = "wasm32"))]
     {
+        // Headless automation server — drive the app over JSON on stdin/stdout
+        // with no GUI (issue #29). `--serve` is the opt-in; everything else
+        // launches the editor.
+        if std::env::args().skip(1).any(|a| a == "--serve") {
+            app::serve();
+            return Ok(());
+        }
         app::run()
     }
 }