libadsb-reader 0.1.48
=====================

SKY PIC API release.

New public API:

    size_t adsb_reader_sky_snapshot(adsb_reader_t *r,
                                    adsb_sky_aircraft_t *out,
                                    size_t capacity);

Returns a caller-owned frozen view of aircraft with a valid ADS-B position
and a receiver contact within the last 30 seconds. The compact structure
contains ICAO24, callsign, latitude/longitude, altitude, ground speed, heading,
valid-field flags, and the last-contact monotonic timestamp.

Standing-data and route enrichment are intentionally excluded; SKY PIC performs
those centrally when the snapshot is uploaded.

A separate mutex-protected SKY PIC cache allows adsb-radar to request snapshots
from its UI thread while decoding continues.

CLI test support:
- adsb-reader --skypic suppresses normal per-message output and prints one
  current SKY PIC snapshot when the reader is stopped (SIGINT/SIGTERM).
