fix: populate MKV Cues index on first screenshots() call

MKV demuxer loads Cues lazily — the AVIndexEntry array stays empty
until a seek triggers it. Do a seek to INT64_MIN at the start of
_screenshots_decode when the index has at most one entry. This gives
screenshots() the full keyframe index for proper seek-based frame
extraction instead of falling back to slow sequential decode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Zesen Qian

src/nvidia_codec/utils/decode.py

@@ -363,6 +363,11 @@ def _screenshots_decode(self, max_interval, start_kts, pic_q):
         """
         max_gap = int(max_interval.total_seconds() / float(self.track._time_base))
 
+        # Trigger index population for containers that load it lazily (e.g. MKV Cues).
+        # If there's at most one index entry, seek to force the demuxer to load its index.
+        if self._keyframe_after(0) is None:
+            self.track.fc.seek_file(self.track.stream, -(2**63), min_ts=-(2**63), max_ts=-(2**63))
+
         kts = start_kts
 
         while True: