libadsb-reader 0.1.51

SKY PIC cache redesign:
- Replaced fixed ADSB_MAX_AIRCRAFT SKY cache array with dynamically growing sorted_collection.
- Numeric ICAO24 is the sorted lookup key; no linear FOR scan and no modulo slot replacement.
- SKY entries remain snapshot-eligible for 30 seconds.
- Entries idle for more than 5 minutes are garbage-collected.
- Cleanup runs at most once per minute during normal decoder updates and again before a snapshot.
- Snapshot writes diagnostics to stderr:
  libadsb-reader: SKY cache: <entries> entries, <eligible> snapshot eligible, <expired> expired removed
- The expired counter includes removals since the previous SKY snapshot/log line.
