adobe-render-bridge: drive AE, Premiere and Media Encoder from Python

Clean-room rebuild of the render half of the Chirps pipeline.

- aerender CLI wrapper with PROGRESS parsing and error extraction that does
  not trust the exit code
- ExtendScript generation with real escaping and a result-file channel back
  from hosts that otherwise report nothing
- Media Encoder watch folder with atomic drop and stability-based completion
- Cross-platform host discovery with version preference and env overrides
- Dry-run mode: the whole pipeline runs, and CI passes, with no Adobe
  software installed
- 23 tests
