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>

src/planemapper/main.py

@@ -1,8 +1,10 @@
 from __future__ import annotations
 
+import dataclasses
 import logging
 import time
 
+import requests
 from PIL import Image, ImageDraw, ImageFont
 
 from planemapper.constants import (
@@ -13,6 +15,7 @@ from planemapper.constants import (
 )
 from planemapper.display import DisplayInterface, WaveshareDisplay
 from planemapper.fetcher import HttpFetcher
+from planemapper.models import Aircraft
 from planemapper.provisioning.config import read as read_config
 from planemapper.renderer.basemap import load as load_basemap
 from planemapper.renderer.projection import MapBounds
@@ -34,9 +37,28 @@ def _make_startup_screen() -> Image.Image:
     return image
 
 
-def _run_one_cycle(renderer: Renderer, fetcher: HttpFetcher, display: DisplayInterface) -> None:
+def _run_one_cycle(
+    renderer: Renderer,
+    fetcher: HttpFetcher,
+    display: DisplayInterface,
+    last_aircraft: list[Aircraft],
+) -> list[Aircraft]:
     t0 = time.monotonic()
-    aircraft_list = fetcher.fetch()
+    stale_needed = False
+    try:
+        fresh = fetcher.fetch()
+    except requests.Timeout:
+        log.warning("fetch timeout — using stale data")
+        fresh = []
+        stale_needed = True
+    else:
+        stale_needed = len(fresh) == 0 and len(last_aircraft) > 0
+
+    if stale_needed:
+        aircraft_list = [dataclasses.replace(a, is_stale=True) for a in last_aircraft]
+    else:
+        aircraft_list = fresh
+
     t1 = time.monotonic()
     image = renderer.render(aircraft_list)
     t2 = time.monotonic()
@@ -52,6 +74,7 @@ def _run_one_cycle(renderer: Renderer, fetcher: HttpFetcher, display: DisplayInt
     )
     if total > RENDER_WARN_S:
         log.warning("render slow: %.1fs > %ds threshold", total, RENDER_WARN_S)
+    return aircraft_list
 
 
 def main() -> None:
@@ -68,6 +91,7 @@ def main() -> None:
     display = WaveshareDisplay()
     startup = _make_startup_screen()
     display.show(startup)
+    last: list[Aircraft] = []
     while True:
-        _run_one_cycle(renderer, fetcher, display)
+        last = _run_one_cycle(renderer, fetcher, display, last)
         time.sleep(REFRESH_INTERVAL_S)