mp4: header creation/modification timestamps, read and write

[jayhf]

Adds read and write support for the ISO/IEC 14496-12 header timestamps (mvhd §8.2.2, tkhd §8.3.2, mdhd §8.4.2), which the muxer previously hardcoded to zero and the demuxer skipped past. All values are in the header epoch, seconds since 1904-01-01 UTC, on both sides — no conversion on intake, so a value read off a file feeds straight back into the muxer.

• Mp4MuxerOptions::creation_time / modification_time stamp all three boxes; track_header_timestamps overrides tkhd/mdhd per stream, with a per-entry media override for a source that dates its media apart from the track carrying it. An entry replaces the pair it names rather than inheriting either half.
• Mp4Demuxer::mvhd_timestamps() / tkhd_timestamps(stream) / mdhd_timestamps(stream) return the new demux::HeaderTimestamps, mirrored on the flat metadata channel as mvhd_* / tkhd_* / mdhd_* keys, emitted only when non-zero.
• Epoch helpers mp4_secs_from_unix_secs / mp4_secs_from_system_time / unix_secs_from_mp4_secs plus MP4_EPOCH_OFFSET_SECS.
• A timestamp past 2040-02-06 in either half promotes its box from version 0 to version 1, per box and automatic.
• Every field the demuxer can report is independently writable. Default output stays byte-identical to before.

15 integration tests, including a byte-patched fixture putting six distinct values in the six fields, and a PATH-gated cross-check that reads back a stamp written by ffmpeg. Note the ffmpeg check skips on CI, since the crate-ci shim installs no extra packages.

[jayhf]

@MagicalTux can you please review this?