[zfleeman]

Closes #129.

A cleanup pass that makes the code shorter and easier to follow. The only change users will notice is that a bad --astreams value now stops the run with an error, instead of being logged and ignored. That matches how #121 handles other bad settings.

What changed

• --astreams parsing (arguments.py): _parse_astreams went from about 50 lines to about 10. It still accepts a comma-separated string or a JSON list from a config file and removes duplicates. Negative, decimal, blank, or non-numeric values now raise a ValueError, and main() shows the message and exits with code 1 (no traceback).
• CODEC_OVERRIDES (twopass.py): now a flat {codec: {second-pass options}} table. No codec used the "both" or "pass1" sections, so the loop is one update() call, and the test that only patched in a fake "both" entry is gone.
• versioning.py: rewritten down to what the update notice needs. The pyproject.toml fallback and the unused package_name= and timeout_s= parameters are gone. Versions are compared by their leading numbers (so 0.4.10 is newer than 0.4.9), replacing a helper that also compared the raw strings. A missing install now catches only PackageNotFoundError, and the comment in main() no longer claims the check never delays startup (it waits up to the 1.5 second timeout).
• Module docstrings: replaced the out-of-date "Functions:" / "Classes:" lists with a one-line summary.
• red(text) helper (twopass.py): replaces the repeated "\033[31m" + dedent(...).strip() + "\033[0m" pattern in twopass.py and __main__.py. windows.py is left alone, since it must not import other modules.
• parse_time(value, duration): replaces timestamp_from_percentage. It returns seconds directly, so clip_times no longer turns a percentage into a timestamp string and parses it back.
• run_pass reuses command_for_display for the failed command, so the error shows a command you can paste into a terminal.
• Smaller items: FILE_SIZE_MULT = 1 / 1024**2; _normalize_amix_args is inlined in get_args; logging.getLogger().setLevel(logging.INFO) moved from import time into main(); the -a default now comes from FALLBACK_AUDIO_BR so the two 96 kbps values can't drift apart.

Testing

• uv run pytest --cov: 296 passed, 99.34% coverage.
• uv run ruff format . && uv run ruff check .: clean.
• Real encode: ff4d bikes.mp4 --from 10% --to 00:00:15 -s 2 produced a 1.98 MB file.
• ff4d bikes.mp4 --astreams "0,-1" --dry-run exits with code 1 and prints Invalid --astreams value '0,-1'. ....