feat: add tkinter GUI export tool for 9 constellation formats

Standalone graphical tool (humeris-gui) that exports constellations to
CSV, GeoJSON, KML, Celestia, Stellarium, Blender, SpaceEngine, KSP, and
Universe Sandbox. Default constellation loads on startup, optional
CelesTrak live data. Background threading for exports and fetches.
31 new tests.

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

Author: pljeroen

packages/core/pyproject.toml

@@ -26,6 +26,7 @@ live = ["sgp4>=2.22"]
 
 [project.scripts]
 humeris = "humeris.cli:main"
+humeris-gui = "humeris.gui:main"
 
 [tool.hatch.build.targets.wheel]
 packages = ["src/humeris"]