review(2-5): code-review pass for per-aircraft drawing — add heading guard, close story

- Add `or 0.0` defensive guard on `aircraft.heading` in `draw_aircraft` per spec (task 1.6)
- Story 2-5 status: review → done
- Sprint status updated: 2-5-per-aircraft-drawing done
- Deferred work: add [2-5] default font 8px readability risk and [2-5] inline arrow geometry constants

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 done, 2-5 review, epic-2 in-progress
+last_updated: 2026-04-22  # 2-1 done, 2-2 done, 2-3 done, 2-4 done, 2-5 done, epic-2 in-progress
 project: planeMapper
 project_key: NOKEY
 tracking_system: file-system
@@ -57,7 +57,7 @@ development_status:
   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: done
-  2-5-per-aircraft-drawing: review
+  2-5-per-aircraft-drawing: done
   2-6-stateful-renderer-and-display-interface: backlog
   2-7-operational-radar-loop-startup-screen-and-systemd-wiring: backlog
   epic-2-retrospective: optional