[codex] Fix oversample repack and transfer regressions

[SashaRX]

Summary

• Disable native xatlas brute-force packing automatically when internalOversample > 1, while keeping the existing cost budget for 1x packs.
• Carry resolved repack atlas dimensions through MeshEntry into GroupedShellTransfer, then scale UV2 margins in pixel terms instead of fixed normalized-space constants.
• Update EXPERIMENTS.md with the follow-up findings and add EditMode regression tests for both fixes.
• Fix two current compile blockers: the scene FBX metrics AnalyzeMesh call and tuple field references in UvShellExtractorTests.

Root Cause

internalOversample = 4 raised the internal xatlas search area from 256x256 to roughly 1024x1024, but the brute-force preflight still allowed the Carousel-sized case because it stayed below the old 500M operation budget. Separately, transfer used normalized UV2 tolerances like 0.005 and 0.01; after xatlas grew resolved atlases past 1k, those constants represented many more pixels than before and could mask extrapolation into neighboring packed regions.

Validation

• git diff --cached --check
• Unity 6000.2.6f2 EditMode tests in a temporary project referencing this package by file: path: 9/9 passed

Notes

The repository checkout does not include the Carousel / Playground / WateringCan FBX suite or prior BenchmarkReports, so model-level repackMs, density spread, and overlap metrics still need to be captured manually in Unity against the benchmark suite.