fix: enforce LC_ALL=C locale and sanitize chapter timestamp ordering

[niStee]

Closes #23

Summary

This PR resolves two related bugs encountered during video playback with ani-skip and mpv:

1. Locale-dependent float parsing failure: On systems with European / non-POSIX locales (e.g. de_DE.UTF-8, fr_FR.UTF-8), printf "%.3f" in ftoi() failed when parsing decimal-dot JSON floats (printf: 151.02: invalid number).
2. Missing chapter timestamp sanity/order check: Corrupted or out-of-order community timestamps from the API (such as ed_start < op_end) previously produced out-of-order chapter files that caused mpv's ffmpeg demuxer to reject the chapter file with Chapter end time ... before start ... and fail avformat_open_input().

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Changes

• Set export LC_ALL=C: Ensures POSIX-standard decimal float formatting, regex, and string transformations regardless of host locale environment.
• Sanitize timestamps in build_options():
  • Validates that start_time < end_time for both Opening and Ending intervals.
  • Ensures Ending strictly follows Opening (ed_start >= op_end). If corrupted, the anomalous segment is safely ignored rather than crashing the metadata demuxer.
  • Writes chapters in chronological order (Opening -> Episode -> Ending).

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Verification

Tested with LC_ALL=de_DE.UTF-8 across:

• Normal episodes (e.g. MAL ID 52991 Ep 1): Valid, sequential Opening, Episode, and Ending chapters generated cleanly.
• Episodes with corrupted ED timestamps (e.g. MAL ID 39551 Ep 1): Inverted ED is safely dropped, Opening skip options and valid metadata are preserved without ffmpeg rejection.

[synacktraa]

Thanks for the PR, I will test and merge it this weekend.