fix: preserve RTree row id semantics

[ddupg]

Summary

• keep RTree scalar index payloads interpreted as logical row ids
• reject fragment coverage recalculation from RTree row-id payloads instead of deriving fragments from physical row addresses
• cover stable-row-id multi-fragment RTree SQL scans where _rowid differs from _rowaddr

Testing

• cargo fmt --all
• cargo test -p lance test_geo_rtree_index
• cargo test -p lance-index --features geo scalar::rtree::tests
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/rtree.rs	94.73%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[ddupg]

@Xuanwo Could you help review this? thanks.