Fix three accuracy bugs in open_geotiff/to_geotiff

[brendancol]

Closes #1081.

Summary

• Windowed reads of PixelIsPoint files were getting a half-pixel coordinate shift (~15m for 1-arc-second DEMs). The non-windowed path already checked raster_type correctly, but the windowed override at line 216 always added the 0.5 offset. Fixed by adding the same check.
• Custom CRS without EPSG codes (local grids, compound CRS) were silently dropped on write because write() only accepted crs_epsg: int. Added crs_wkt pass-through to build_geo_tags, which stores the WKT in GeoAsciiParamsTag and sets the CRS GeoKey to 32767 (user-defined).
• open_geotiff converts nodata sentinels to NaN, but to_geotiff wasn't converting them back. The written file would have NaN pixels but a GDAL_NODATA tag still pointing at the original sentinel (e.g. -9999). Fixed by restoring sentinel values from NaN before writing.

Test plan

• 10 new tests in test_accuracy_1081.py covering all three bugs
• 344 existing geotiff tests still pass (0 failures)
• Verify windowed read coords match full-read slice for both PixelIsArea and PixelIsPoint
• Verify GeoAsciiParamsTag written for non-EPSG WKT CRS
• Verify raw pixel data has sentinel values (not NaN) after round-trip