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_colours.py

@@ -1,2 +1,33 @@
-def test_placeholder() -> None:
-    pass
+from __future__ import annotations
+
+import pytest
+
+from planemapper.constants import ALTITUDE_COLOURS
+from planemapper.renderer.colours import altitude_to_colour
+
+
+@pytest.mark.parametrize(
+    "altitude_ft,expected",
+    [
+        (0, ALTITUDE_COLOURS[0]),  # surface → GREEN
+        (1500, ALTITUDE_COLOURS[0]),  # boundary inclusive → GREEN
+        (1501, ALTITUDE_COLOURS[1]),  # just above → BLUE
+        (5000, ALTITUDE_COLOURS[1]),  # boundary inclusive → BLUE
+        (5001, ALTITUDE_COLOURS[2]),  # just above → YELLOW
+        (10000, ALTITUDE_COLOURS[2]),  # boundary inclusive → YELLOW
+        (10001, ALTITUDE_COLOURS[3]),  # just above → RED
+        (20000, ALTITUDE_COLOURS[3]),  # boundary inclusive → RED
+        (20001, ALTITUDE_COLOURS[4]),  # just above → BLACK
+        (35000, ALTITUDE_COLOURS[4]),  # boundary inclusive → BLACK
+        (35001, ALTITUDE_COLOURS[5]),  # just above → WHITE
+        (99999, ALTITUDE_COLOURS[5]),  # max band → WHITE
+        (100000, ALTITUDE_COLOURS[5]),  # beyond max → WHITE (fallback)
+    ],
+)
+def test_altitude_to_colour(altitude_ft: int, expected: tuple[int, int, int]) -> None:
+    assert altitude_to_colour(altitude_ft) == expected
+
+
+def test_all_six_colours_reachable() -> None:
+    results = {altitude_to_colour(alt) for alt in [0, 2000, 7500, 15000, 25000, 40000]}
+    assert results == set(ALTITUDE_COLOURS)