fix(viewer): use pre-transformed WCS bounds for block group spatial indexing

[mlightcad]

Summary

• Rename entity bounding-box metadata from box/boxes to wcsBbox/wcsChildBoxes across three-renderer and cad-simple-viewer for clearer WCS semantics.
• Fix AcTrView2d.handleGroup double-transform bug: AcDbRenderingCache.draw already applies the INSERT matrix via applyMatrix, so spatial indexing must use the pre-transformed WCS bounds directly instead of multiplying group.matrix again.
• Add dev-only consistency assertions (AcTrGroupWcsBboxAssert) and unit tests covering entity/group WCS bbox updates, layout spatial registration, and handleGroup indexing behavior.

Test plan

• pnpm test -- --testPathPatterns="AcTr(Entity|Group|Line|LineSegments|MText|Point|Polygon|Shape|Image|Layout|View2d)" (60 tests passed)
• Open a drawing with block references (INSERT) and verify window/crossing selection hits nested entities correctly
• Confirm zoom extents and spatial queries still behave for mixed layer block inserts