feat(3-1): stale state detection and dimmed display

When fetch times out or returns empty after prior data, retain last aircraft
with is_stale=True and render them as black outlines so the display stays
informative rather than blank or crashing.

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

tests/test_stale.py

@@ -0,0 +1,71 @@
+from __future__ import annotations
+
+import dataclasses
+import pathlib
+from unittest.mock import MagicMock
+
+import pytest
+import requests
+from PIL import Image
+
+from planemapper.display import NullDisplay
+from planemapper.main import _run_one_cycle
+from planemapper.models import Aircraft
+from planemapper.renderer.projection import MapBounds
+from planemapper.renderer.renderer import Renderer
+
+FIXTURE_DIR = pathlib.Path(__file__).parent / "fixtures"
+
+
+@pytest.fixture
+def renderer(monkeypatch: pytest.MonkeyPatch) -> Renderer:
+    monkeypatch.setattr(
+        "planemapper.renderer.airspace.AIRSPACE_PATH",
+        FIXTURE_DIR / "airspace_sample.geojson",
+    )
+    base_map = Image.new("RGB", (800, 480), color=(255, 255, 255))
+    bounds = MapBounds(home_lat=53.0, home_lon=-6.0, radius_nm=100.0)
+    return Renderer(base_map, bounds)
+
+
+def _make_aircraft(icao: str = "ABC123") -> Aircraft:
+    return Aircraft(icao=icao, lat=53.1, lon=-6.1, altitude_ft=10000, callsign="TST1")
+
+
+def test_timeout_returns_stale_last_aircraft(renderer: Renderer) -> None:
+    display = NullDisplay()
+    last = [_make_aircraft()]
+    mock_fetcher = MagicMock()
+    mock_fetcher.fetch.side_effect = requests.Timeout
+    result = _run_one_cycle(renderer, mock_fetcher, display, last)
+    assert len(result) == 1
+    assert result[0].is_stale is True
+
+
+def test_empty_fetch_returns_stale_when_had_previous(renderer: Renderer) -> None:
+    display = NullDisplay()
+    last = [_make_aircraft()]
+    mock_fetcher = MagicMock()
+    mock_fetcher.fetch.return_value = []
+    result = _run_one_cycle(renderer, mock_fetcher, display, last)
+    assert len(result) == 1
+    assert result[0].is_stale is True
+
+
+def test_stale_aircraft_rendered_without_exception(renderer: Renderer) -> None:
+    display = NullDisplay()
+    stale_aircraft = dataclasses.replace(_make_aircraft(), is_stale=True)
+    mock_fetcher = MagicMock()
+    mock_fetcher.fetch.side_effect = requests.Timeout
+    result = _run_one_cycle(renderer, mock_fetcher, display, [stale_aircraft])
+    assert result[0].is_stale is True
+
+
+def test_stale_cycle_completes_without_crash(renderer: Renderer) -> None:
+    display = NullDisplay()
+    last = [_make_aircraft()]
+    mock_fetcher = MagicMock()
+    mock_fetcher.fetch.side_effect = requests.Timeout
+    # Should complete without any exception
+    result = _run_one_cycle(renderer, mock_fetcher, display, last)
+    assert isinstance(result, list)