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>

tests/fixtures/airspace_sample.geojson

@@ -1 +1,21 @@
-{"type": "FeatureCollection", "features": []}
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "geometry": {
+        "type": "Polygon",
+        "coordinates": [
+          [
+            [-6.5, 53.3],
+            [-5.5, 53.3],
+            [-5.5, 53.7],
+            [-6.5, 53.7],
+            [-6.5, 53.3]
+          ]
+        ]
+      },
+      "properties": {"name": "Test Airspace"}
+    }
+  ]
+}

tests/test_airspace.py

@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+import pathlib
+
+import pytest
+from PIL import Image
+
+from planemapper.renderer.airspace import draw_airspace
+from planemapper.renderer.overlay import draw_home_marker
+from planemapper.renderer.projection import MapBounds
+
+FIXTURE_DIR = pathlib.Path(__file__).parent / "fixtures"
+
+
+@pytest.fixture
+def white_image() -> Image.Image:
+    return Image.new("RGB", (800, 480), color=(255, 255, 255))
+
+
+@pytest.fixture
+def bounds() -> MapBounds:
+    return MapBounds(home_lat=53.0, home_lon=-6.0, radius_nm=100.0)
+
+
+def test_home_marker_draws_red_cross(white_image: Image.Image, bounds: MapBounds) -> None:
+    draw_home_marker(white_image, bounds)
+    # Centre pixel should be red (COLOUR_HOME_MARKER)
+    assert white_image.getpixel((400, 240)) == (255, 0, 0)
+
+
+def test_airspace_drawn_without_exception(
+    white_image: Image.Image,
+    bounds: MapBounds,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(
+        "planemapper.renderer.airspace.AIRSPACE_PATH",
+        FIXTURE_DIR / "airspace_sample.geojson",
+    )
+    draw_airspace(white_image, bounds)  # should not raise
+
+
+def test_airspace_missing_file_no_exception(
+    white_image: Image.Image,
+    bounds: MapBounds,
+    tmp_path: pathlib.Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(
+        "planemapper.renderer.airspace.AIRSPACE_PATH",
+        tmp_path / "nonexistent.geojson",
+    )
+    draw_airspace(white_image, bounds)  # should not raise — logs WARNING instead