feat: add raster read planning statistics

[snkmcb]

Summary

• coalesce adjacent uncompressed TIFF segment reads up to 64 KiB
• expose requested bytes, fetched bytes, request count, and amplification through RasterReadStatistics
• avoid unnecessary segment copies when no predictor is active
• update regression coverage and reader documentation

Validation

• usdGeoCore: 146 checks passed
• usdRasterCore: 892 checks passed
• GeoTIFF metadata tests passed
• GeoTIFF pixel tests passed

The libtiff-enabled configuration could not be regenerated in this environment because TIFF_LIBRARY and TIFF_INCLUDE_DIR were unavailable.