Add software adaptive gain control, ported from dump1090-fa

[tytech039]

This ports dump1090-fa's adaptive gain feature (README.adaptive-gain.md, adaptive.c) to dump978. Two independent modes, both off by default and available only with --sdr:

• --adaptive-range (dynamic range mode): tracks the noise floor and holds the highest gain that still meets a target dynamic range (default: 60% of the SDR's gain range in dB, ~30dB for an RTLSDR).
• --adaptive-burst (burst mode): backs the gain off when loud, message-length, undecodable bursts suggest receiver overload, and raises it again once things quieten down.

Implementation notes

• The full set of dump1090-fa tuning options is exposed under the same names and defaults (--adaptive-min-gain, --adaptive-max-gain, --adaptive-duty-cycle, and the per-mode knobs). Adaptive gain requires manual gain control and is rejected in combination with --sdr-auto-gain.
• Gain is stepped through a table built from the SoapySDR gain range, falling back to 1dB steps if the device reports no step size.
• Magnitudes are fed from a new SampleConverter::ConvertMag16 output; decoded message extents are excluded from burst detection as in dump1090. All adaptive work runs on the SDR rx thread, serialized with readStream.
• Burst detection is adjusted for UAT timing rather than copied verbatim: dump1090 counts loud-window runs of 2–5 windows (80–200µs), matching Mode S frames. UAT downlink frames are 276/420 bits at 1.041667Mbps (~265µs/~403µs), so this accepts runs of 5–13 windows; shorter runs (e.g. DME pulses) and longer ones (ground uplink frames, continuous interference) are ignored. A decoded message's samples are also carried across the adaptive feed boundary so a message clipped at a buffer edge isn't miscounted as an undecoded burst on the next call.
• Gain changes and a once-a-minute status line are logged to stderr; a README section documents the feature.

Licensing

adaptive.h/adaptive.cc/adaptive_tests.cc retain dump1090's GPLv2+ license headers (copyright FlightAware in both projects). Note this differs from dump978's 2-clause BSD license — happy to adjust if you'd prefer to relicense your own code under BSD here.

Testing

adaptive_tests.cc covers the ported logic and runs under make test alongside fec_tests.