Add pytest suite and CI (roadmap A1)

37 headless tests covering the logic that is cheap to break:
- split planner: plan_disc_split, chapter helpers (whole-fit, multi-disc
  partition, single-movie chapter/even-cut split, contiguous coverage)
- High-profile fill-disc bitrate math (disc count + bounds + capacity)
- ffmpeg command generation: profiles, text/image hardsub, 2-pass, nice,
  progress-to-file, even pad/crop + positive offsets, -ss seek, and the
  AC3 448k audio-clamp regression
- ffprobe subtitle detection: text/image classification, tags/disposition

tests/conftest.py makes the in-tree package importable and installs the
gettext _ builtin so modules run headless. Forgejo Actions workflow runs
the suite on push/PR. Update CLAUDE.md's testing section accordingly.

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

.gitignore

@@ -5,6 +5,7 @@ __pycache__/
 build/
 dist/
 *.egg-info/
+.pytest_cache/
 
 # Compiled translations (generated from po/ at build time)
 src/devedeng/data/locale/