[ifnull]

What was wrong

max_alt_agl_ft resolved height above ground from canonical.alt_baro_ft alone.
Remote ID carries no barometric altitude, so a drone track's alt_baro_ft is
always null and the gate returned false unconditionally — every AGL-bounded
alert rule was unsatisfiable for drones.

Both drone rules in our own docs (drone_conflict, drone_nearby) carry
max_alt_agl_ft, so in practice they could only ever fire on manned aircraft.
That is what was happening in the field: drone_nearby was on, triggered by a
helicopter.

It failed in the worst possible way — silently. No error, no log line, no
unhealthy receiver. Drones appeared correctly in drone_count and
nearest_drone the entire time, so the pipeline looked healthy and only the
alert never came.

The fix

AGL now resolves most-direct-first:

1. drone.agl_ft — Remote ID broadcasts true height above takeoff, which is
   better than the MSL-minus-watchpoint-elevation approximation we use for
   aircraft.
2. alt_geom_ft — for a drone whose Location message omits AGL.
3. alt_baro_ft minus ground elevation — unchanged for manned aircraft.

Manned-aircraft matching is byte-identical. Geometric altitude is
ellipsoid-referenced, so folding it into the MSL path would shift AGL by the
local geoid separation and quietly move empirically tuned thresholds. Left
deliberately out of scope.

Verification

Reproduced against tests/fixtures/remoteid_basic.json, then confirmed
end-to-end on a live Remote ID feed: a drone at 0.72 nm and 279 ft AGL, with
alt_baro_ft: null, now matches drone_nearby, publishes the alert, flips the
Home Assistant binary_sensor, and fires the notification automation. The same
track matched nothing before the fix.

Six regression tests added, including one pinning unchanged manned-aircraft
behavior.

Also in this PR

• Example thresholds widened and reconciled. config.example.yaml and
  docs/automations.example.yaml gave different values for the same
  drone_nearby rule (0.5/1000 vs 0.34/1640), and both distance bounds
  excluded a drone under a nautical mile out. A too-tight bound fails silently —
  the same class of problem as the bug itself.
• scripts/watch_rid.py — prints what the feed actually carries and labels
  each drone ALERTABLE vs BASIC-ID-ONLY, so "transmitter sent no position"
  is distinguishable from "threshold excluded it".
• Docs warning — an automation triggered by an alert entity must not read
  sensor.airspace_nearest_drone. On reconnect Home Assistant sets up the
  retained binary_sensor discovery first, so the alert entity reaches on
  while nearest_drone is still unavailable; the condition then fails every
  restart-triggered edge and looks exactly like the alert never firing.
• Version bump to 1.1.6 — required by the version-guard CI job, which
  needs pyproject.toml and addon/config.yaml to agree and be ahead of the
  last tag.

Reviewer notes

• No config schema change and no payload contract change — schema_version is
  untouched.
• Until this is tagged and the add-on image updated, drone alerts remain broken
  in the published release.

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