All three tool handlers (compile_scad, render_stl, lint_scad) were
calling async core functions without await from synchronous def bodies.
This caused FastMCP to receive a coroutine object instead of a dict,
which then failed JSON serialization with:
  Object of type coroutine is not JSON serializable

Fix: convert tool handlers to async def and await the underlying
compile_scad / render_stl / lint_scad calls before passing to _json().

Tested via stdio_client — lint_scad now returns clean JSON result.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>