feat(story-2.3): implement home marker and airspace outline rendering

Adds draw_home_marker() in overlay.py (red cross at projected home
position) and draw_airspace() in airspace.py (GeoJSON Polygon
boundaries in blue, graceful FileNotFoundError handling). Includes
airspace fixture and three tests covering all acceptance criteria.
All 70 tests pass; ruff check and format clean.

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

src/planemapper/renderer/airspace.py

@@ -1 +1,30 @@
-# stub
+from __future__ import annotations
+
+import json
+import logging
+
+from PIL import Image, ImageDraw
+
+from planemapper.constants import AIRSPACE_PATH, COLOUR_AIRSPACE
+from planemapper.renderer.projection import MapBounds, project
+
+log = logging.getLogger(__name__)
+
+
+def draw_airspace(image: Image.Image, bounds: MapBounds) -> None:
+    try:
+        data = json.loads(AIRSPACE_PATH.read_text(encoding="utf-8"))
+    except FileNotFoundError:
+        log.warning("airspace.geojson not found at %s — skipping airspace overlay", AIRSPACE_PATH)
+        return
+    draw = ImageDraw.Draw(image)
+    for feature in data.get("features", []):
+        geom = feature.get("geometry", {})
+        if geom.get("type") != "Polygon":
+            continue
+        coords = geom.get("coordinates", [[]])[0]
+        if len(coords) < 2:
+            continue
+        # GeoJSON is [lon, lat] — reverse at parse boundary
+        points = [project(lat, lon, bounds) for lon, lat in coords]
+        draw.line(points, fill=COLOUR_AIRSPACE, width=2)

src/planemapper/renderer/overlay.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+from PIL import Image, ImageDraw
+
+from planemapper.constants import COLOUR_HOME_MARKER
+from planemapper.renderer.projection import MapBounds, project
+
+
+def draw_home_marker(image: Image.Image, bounds: MapBounds) -> None:
+    cx, cy = project(bounds.home_lat, bounds.home_lon, bounds)
+    draw = ImageDraw.Draw(image)
+    draw.line([(cx - 10, cy), (cx + 10, cy)], fill=COLOUR_HOME_MARKER, width=3)
+    draw.line([(cx, cy - 10), (cx, cy + 10)], fill=COLOUR_HOME_MARKER, width=3)