Add transponder outages (frac_adsb_outage)

[jehanazad]

Summary

The simulator never stopped ADS-B. Every has_adsb aircraft was tagged in every node frame and pushed to the server every second for its whole life, so the backend's "known-track hold" (a node track claimed to an ADS-B hex staying claimed after the broadcast stops) had no way to be exercised against simulated traffic.

This adds a transponder-outage model: an ADS-B aircraft goes silent mid-flight and comes back. has_adsb and adsb_hex are deliberately untouched — the aircraft still HAS a transponder, it is simply not broadcasting, which is what separates this from a dark target.

Default is 0.0 (off): nothing changes for anyone who does not set the knob.

Changes

• SimulatedAircraft: adsb_outage_start_s / adsb_outage_end_s (world-clock seconds), sim_now_s (clock refreshed each step()), and an adsb_silent property.
• SimulationWorld.frac_adsb_outage (default 0.0) — the fraction OF the ADS-B population that gets an outage. Rolled at spawn: start 20–120 s after spawn, duration 60–240 s. Those bounds are module constants: long enough to age past the backend's 45 s fix cap and 60 s map expiry, short enough to recur several times in a 20-minute capture.
• schedule_adsb_outages() rolls live ADS-B aircraft that have no window yet (start 5–60 s out), so raising the knob at runtime does not have to wait for a fleet turnover. Aircraft that already carry a window are skipped, so a 5 s config poll cannot re-roll the same aircraft forever.
• generate_detections_for_node: while silent, the ADS-B list gets a None slot — exactly what a dark aircraft gets. Delay/Doppler/SNR and the miss model are unchanged, so the echo is still there.
• get_aircraft_summary reports adsb_silent; build_ground_truth_payload passes it through; build_adsb_push_payload skips silent aircraft (that push IS the broadcast).
• _poll_simulation_config applies cfg["frac_adsb_outage"] (default 0.0) alongside frac_dark, and re-rolls in-flight aircraft when it is on.

Test coverage

New tests/test_adsb_outage.py: knob at 1.0 vs 0.0, dark aircraft never scheduled, half-open silence window, runtime re-roll (including the idempotence of repeated polls), tag present before/after and None during, radar detections unaffected while silent, summary flag, ground-truth passthrough with a legacy-payload default, and push-payload skipping. tests/test_dual_fraction.py's stub world gained the new attribute/method.

Full lib suite: 128 passed, 5 skipped.

Review notes

• The outage is intentionally orthogonal to frac_dark: it is a fraction of the ADS-B aircraft, not a spawn-type roll, so it does not participate in the frac_* sum.
• adsb_silent reads sim_now_s rather than a back-reference to the world; step() refreshes it for every aircraft, and _spawn_aircraft seeds it.

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