front/engine split, ds4 + media tooling, gemma-4 native tools; ignore runtime artifacts

- frontproxy: torch-free front proxy + per-vendor engine supervisor with auth,
  localhost binding, model routing; Ctrl-C now force-kills engines (own session +
  PDEATHSIG, SIGKILL of engine process groups, watchdog on hung drain)
- gemma-4 tool calling: prompt via native tools= template, parse call:NAME{...}
  into tool_calls, honour generation_config EOS so it stops instead of looping
- ds4 external worker, parler/expressive TTS backends, video editor tooling
- --debug-requests: full client<->API request/response logging + live snapshots
- stop tracking runtime artifacts (video_editor/sessions/, tools/coderai_media/)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>