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>

src/planemapper/display.py

@@ -1,7 +1,12 @@
+from __future__ import annotations
+
+import logging
 from typing import Protocol
 
 from PIL import Image
 
+log = logging.getLogger(__name__)
+
 
 class DisplayInterface(Protocol):
     def show(self, image: Image.Image) -> None: ...
@@ -9,4 +14,9 @@ class DisplayInterface(Protocol):
 
 class NullDisplay:
     def show(self, image: Image.Image) -> None:
-        pass
+        log.debug("NullDisplay.show: %dx%d", image.width, image.height)
+
+
+class WaveshareDisplay:
+    def show(self, image: Image.Image) -> None:
+        raise NotImplementedError