Implement story 2.2: coordinate projection and base map loading

Add MapBounds dataclass and equirectangular project() function in
projection.py, basemap.load() forcing pixels into memory via .copy(),
and full test coverage for both modules (4 new tests). All quality
gates pass: 67 tests, ruff clean.

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

src/planemapper/renderer/basemap.py

@@ -1 +1,9 @@
-# stub
+from __future__ import annotations
+
+from PIL import Image
+
+from planemapper.constants import BACKGROUND_PATH
+
+
+def load() -> Image.Image:
+    return Image.open(BACKGROUND_PATH).copy()