Emit emergency-squawk events from local ADS-B adapter (M2)

[kninetimmy]

Closes out the last partial Milestone 2 (Local RF baseline) deliverable from PRD §32: emergency-squawk templates. The local ADS-B adapter already recognized emergency squawks but only tagged the track — it never surfaced a discrete event. It now emits a critical EventRecord on the onset edge, rendered by the existing EventFeed.

What changed

• ThrottleGate.admit() now returns Admission(publish, emergency_onset). The not-emergency → 7500/7600/7700 (or explicit transponder emergency) edge was already computed for the immediate-publish bypass; it's now exposed rather than discarded.
• On that edge, local_adsb_records() yields a critical EventRecord (event_type=emergency_squawk, severity=critical, subject_id=track, geometry=last position, local_rf provenance copied from the track). Fires once per onset — a steady stream of the same squawk does not re-fire.
• EMERGENCY_SQUAWK_MEANINGS maps each code to plain language (e.g. 7700 → general emergency).
• No frontend change — EventFeed already renders event_type + summary.

This is the event substrate the M4 alert-rule engine will later consume — not the rule engine itself.

Tests / verification

• New unit tests: onset detection, generator emits the event, steady-state no-refire.
• New integration test (aircraft_emergency.json) exercises the full path adapter → MQTT → backend → websocket and asserts a event frame with critical severity, 7700 in the summary, Point geometry, and local_rf: true.
• Adjusts one pre-existing test whose item ordering shifts because the standard fixture already carries a 7700 aircraft (so the onset event now correctly fires for it too).
• scripts/check.sh green — ruff + mypy (strict) + 125 tests passed.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request implements the M2 emergency-squawk event template. It updates the local ADS-B adapter to detect when an aircraft transitions into an emergency state (e.g., squawking 7500, 7600, or 7700) and emits a discrete critical EventRecord on the stream. To support this, ThrottleGate.admit now returns an Admission named tuple indicating both whether to publish and whether an emergency onset occurred. New integration and unit tests have been added to verify this behavior. There are no review comments, so I have no feedback to provide.

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