Blu-ray SUP export: fix invalid PTS/DTS on exported segments

[niksedk]

Fixes #10219.

What was wrong

BluRaySupPicture.CreateSupFrame (a port of BDSup2Sub's writer) mixed up the timestamp fields:

• The ODS segment got PTS=0 with the decode-model value in DTS — the exact PTS 0, DTS 13097155 segment from the issue, violating "decoding must finish before presentation" (DTS ≤ PTS).
• The first END segment inherited the ODS's PTS=0.
• dts = (uint)(pts - decodeTimes) underflowed to a huge value for captions starting in the first ~150 ms.
• WDS/PDS carried assorted offset PTS values; the original BDSup2Sub code writes DTS=0 on every segment.

Tools like SupMover shift the PTS of every segment when applying a delay, so the zeroed/garbled fields made every caption trigger "starts before the full delay amount" on negative delays (MonoS/SupMover#30).

The fix

Every segment of a display set now carries the display set's presentation time as PTS, and DTS stays 0 ("unset"). Rationale, checked against the ecosystem:

• This is how .sup files extracted from retail discs look (per the segment traces in the SupMover issue) — uniform PTS per display set, empty DTS.
• ffmpeg's sup demuxer has an explicit code path: "Many files have DTS set to 0 for all packets, so assume 0 means unset".
• SupMover shifts each segment's PTS uniformly, which now works.
• Muxers (tsMuxeR etc.) re-derive PES decode timing from the PCS when authoring an m2ts; the standalone .sup DTS field is informational.
• Uniform PTS within a display set also keeps packets monotonic, so ffmpeg -c copy remuxing produces no "non-monotonous DTS" warnings (staggered per-segment PTS would).

Verification

Generated test sups via libse (caption at t=0, small gaps, and a 900×300 noisy bitmap forcing a 5-packet ODS chain):

• Segment validator (python, parses every PG header): DTS=0 everywhere, DTS ≤ PTS, uniform PTS per display set, monotonic across sets, clean PCS…END structure. ✔
• ffmpeg 4.4 remux to mkv: all 44 segments copied, zero warnings, timestamps intact. ✔
• ffmpeg PGS decode (overlay onto black + per-frame signalstats): captions appear and clear at exactly the authored times, including the multi-packet ODS. ✔
• SupMover (built from source): --delay -335 on a shifted file → zero warnings, all display sets moved exactly 335 ms, output still passes the validator. The only warnings left are for a caption genuinely starting at t=0, which is correct clamping. ✔
• SE roundtrip: BluRaySupParser re-imports all captions with correct times, sizes, and forced flags. ✔
• All 997 unit tests pass.

🤖 Generated with Claude Code