feat(2-6): stateful Renderer, DisplayInterface, and pipeline smoke test

Implements story 2-6: Renderer class with per-aircraft trail history
(deque capped at TRAIL_MAX_DOTS=5), NullDisplay with DEBUG logging,
WaveshareDisplay stub, and end-to-end pipeline smoke test. All 96
tests pass; ruff check and format clean.

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

tests/test_renderer.py

@@ -1,2 +1,61 @@
-def test_placeholder() -> None:
-    pass
+from __future__ import annotations
+
+import pathlib
+
+import pytest
+from PIL import Image
+
+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 white_base_map() -> 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)
+
+
+@pytest.fixture
+def renderer(
+    white_base_map: Image.Image, bounds: MapBounds, monkeypatch: pytest.MonkeyPatch
+) -> Renderer:
+    monkeypatch.setattr(
+        "planemapper.renderer.airspace.AIRSPACE_PATH",
+        FIXTURE_DIR / "airspace_sample.geojson",
+    )
+    return Renderer(white_base_map, bounds)
+
+
+def _aircraft(icao: str = "ABC123", **kwargs) -> Aircraft:
+    defaults = {"lat": 53.1, "lon": -6.1}
+    defaults.update(kwargs)
+    return Aircraft(icao=icao, **defaults)
+
+
+def test_render_returns_800x480(renderer: Renderer) -> None:
+    result = renderer.render([])
+    assert isinstance(result, Image.Image)
+    assert result.size == (800, 480)
+
+
+def test_trail_accumulated_across_renders(renderer: Renderer) -> None:
+    ac = _aircraft(icao="ABC123")
+    renderer.render([ac])
+    renderer.render([ac])
+    assert "ABC123" in renderer._trails
+    assert len(renderer._trails["ABC123"]) == 2
+
+
+def test_absent_aircraft_trail_retained(renderer: Renderer) -> None:
+    ac = _aircraft(icao="ABC123")
+    renderer.render([ac])
+    renderer.render([])  # aircraft absent
+    assert "ABC123" in renderer._trails
+    assert len(renderer._trails["ABC123"]) == 1