feat: Make pip install work without system GDAL; fix CI

[mmann1123]

• Make osgeo imports lazy in 4 files (gdal_.py, io.py, topo.py, web.py) so that import geowombat no longer requires GDAL Python bindings. Users without GDAL get a clear error with per-OS install instructions
  only when calling the few functions that need it.
• Pin stackstac>=0.5.1 to fix numpy 2.x incompatibility (NEP 50)
• Add Tests-result CI job for branch protection compatibility
• Update install docs to reflect that core features work without GDAL

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