feat(2-4): implement altitude colour bands and aircraft type icons

Implements altitude_to_colour() mapping altitude bands to the 6 e-ink
palette colours, and classify_aircraft_type() resolving ADS-B category,
callsign prefix, and altitude fallback to AircraftType enum. Adds 21
new tests (13 parametrised boundary cases + 8 icon classification);
89 tests total, all quality gates green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_icons.py

@@ -1,2 +1,45 @@
-def test_placeholder() -> None:
-    pass
+from __future__ import annotations
+
+from planemapper.models import Aircraft
+from planemapper.renderer.icons import AircraftType, classify_aircraft_type
+
+
+def _aircraft(**kwargs) -> Aircraft:
+    defaults = {"icao": "ABC123", "lat": 53.0, "lon": -6.0}
+    defaults.update(kwargs)
+    return Aircraft(**defaults)
+
+
+def test_category_a1_returns_ga_light() -> None:
+    ac = _aircraft(category="A1")
+    assert classify_aircraft_type(ac) == AircraftType.GA_LIGHT
+
+
+def test_category_a7_returns_helicopter() -> None:
+    ac = _aircraft(category="A7")
+    assert classify_aircraft_type(ac) == AircraftType.HELICOPTER
+
+
+def test_ba_callsign_returns_commercial() -> None:
+    ac = _aircraft(callsign="BAW123")
+    assert classify_aircraft_type(ac) == AircraftType.COMMERCIAL
+
+
+def test_altitude_below_10000_returns_ga_light() -> None:
+    ac = _aircraft(altitude_ft=5000)
+    assert classify_aircraft_type(ac) == AircraftType.GA_LIGHT
+
+
+def test_altitude_18000_returns_private_jet() -> None:
+    ac = _aircraft(altitude_ft=18000)
+    assert classify_aircraft_type(ac) == AircraftType.PRIVATE_JET
+
+
+def test_altitude_38000_returns_airliner() -> None:
+    ac = _aircraft(altitude_ft=38000)
+    assert classify_aircraft_type(ac) == AircraftType.AIRLINER
+
+
+def test_category_a3_returns_commercial() -> None:
+    ac = _aircraft(category="A3")
+    assert classify_aircraft_type(ac) == AircraftType.COMMERCIAL