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>

.forgejo/workflows/tests.yml

@@ -0,0 +1,28 @@
+name: tests
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  pytest:
+    runs-on: docker
+    container:
+      image: debian:bookworm
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: |
+          apt-get update
+          # PyGObject + GTK introspection are needed because the modules under
+          # test import gi.repository at import time; the tests themselves run
+          # headless (no display, no encoding).
+          apt-get install -y --no-install-recommends \
+            python3 python3-pip python3-gi gir1.2-gtk-3.0 \
+            python3-cairo python3-gi-cairo
+          pip3 install --break-system-packages pytest
+
+      - name: Run test suite
+        run: python3 -m pytest