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>

src/planemapper/renderer/colours.py

@@ -1 +1,10 @@
-# stub
+from __future__ import annotations
+
+from planemapper.constants import ALTITUDE_BANDS_FT, ALTITUDE_COLOURS
+
+
+def altitude_to_colour(altitude_ft: int) -> tuple[int, int, int]:
+    for i, band in enumerate(ALTITUDE_BANDS_FT):
+        if altitude_ft <= band:
+            return ALTITUDE_COLOURS[i]
+    return ALTITUDE_COLOURS[-1]

src/planemapper/renderer/icons.py

@@ -1 +1,71 @@
-# stub
+from __future__ import annotations
+
+from enum import Enum
+
+from planemapper.models import Aircraft
+
+
+class AircraftType(Enum):
+    GA_LIGHT = "ga_light"
+    COMMERCIAL = "commercial"
+    PRIVATE_JET = "private_jet"
+    AIRLINER = "airliner"
+    HELICOPTER = "helicopter"
+    MILITARY = "military"
+    UNKNOWN = "unknown"
+
+
+_AIRLINE_PREFIXES: frozenset[str] = frozenset(
+    {
+        "BAW",
+        "EIN",
+        "RYR",
+        "EZY",
+        "THY",
+        "DLH",
+        "AFR",
+        "IBE",
+        "KLM",
+        "UAE",
+        "SWR",
+        "AAL",
+        "UAL",
+        "DAL",
+        "SAS",
+        "TAP",
+        "VLG",
+        "NOS",
+        "WZZ",
+        "AEA",
+        "NAX",
+        "FIN",
+        "CSN",
+        "CCA",
+    }
+)
+
+_CATEGORY_MAP: dict[str, AircraftType] = {
+    "A1": AircraftType.GA_LIGHT,
+    "A2": AircraftType.GA_LIGHT,
+    "A3": AircraftType.COMMERCIAL,
+    "A4": AircraftType.COMMERCIAL,
+    "A5": AircraftType.COMMERCIAL,
+    "A7": AircraftType.HELICOPTER,
+    "B1": AircraftType.MILITARY,
+    "B2": AircraftType.MILITARY,
+    "B3": AircraftType.MILITARY,
+    "B4": AircraftType.MILITARY,
+}
+
+
+def classify_aircraft_type(aircraft: Aircraft) -> AircraftType:
+    if aircraft.category and aircraft.category in _CATEGORY_MAP:
+        return _CATEGORY_MAP[aircraft.category]
+    callsign = aircraft.callsign.strip()
+    if len(callsign) >= 3 and callsign[:3].upper() in _AIRLINE_PREFIXES:
+        return AircraftType.COMMERCIAL
+    if aircraft.altitude_ft < 10000:
+        return AircraftType.GA_LIGHT
+    if aircraft.altitude_ft < 30000:
+        return AircraftType.PRIVATE_JET
+    return AircraftType.AIRLINER