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>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -35,7 +35,7 @@
 # - Dev moves story to 'review', then runs code-review (fresh context, different LLM recommended)
 
 generated: 2026-04-22
-last_updated: 2026-04-22  # 2-1 done, 2-2 done, 2-3 done, 2-4 ready-for-dev, epic-2 in-progress
+last_updated: 2026-04-22  # 2-1 done, 2-2 done, 2-3 done, 2-4 review, epic-2 in-progress
 project: planeMapper
 project_key: NOKEY
 tracking_system: file-system
@@ -56,7 +56,7 @@ development_status:
   2-1-aircraft-data-model-and-fetcher: done
   2-2-coordinate-projection-and-base-map-loading: done
   2-3-home-marker-and-airspace-outlines: done
-  2-4-altitude-colour-bands-and-aircraft-type-icons: ready-for-dev
+  2-4-altitude-colour-bands-and-aircraft-type-icons: review
   2-5-per-aircraft-drawing: backlog
   2-6-stateful-renderer-and-display-interface: backlog
   2-7-operational-radar-loop-startup-screen-and-systemd-wiring: backlog