Improve RealityKit prim-pick resolution for coarse imported hierarchies

[elkraneo]

Summary

RealityKit picking and post-process outlines are now stable in StageView, but prim recovery is still heuristic when imported assets collapse into generic nodes such as merged_1.

Current behavior

• macOS and gesture picking are working
• post-process outline is stable across repeated selections
• imported meshes are outlined through MeshResource.contents
• coarse hits are remapped to semantic paths with shared StageView heuristics

Remaining problem

When Entity(contentsOf:) collapses the imported graph, reverse picking still often resolves to assembly-level or material-partition prims (..._Blue, ..._Decals) instead of the most useful authored prim for editing.

Follow-up goals

• improve ordered-hit ranking so generic parent hits do not dominate small-detail selection
• evaluate whether pick proxies or consumer-provided resolver hooks should be the preferred exact-selection path
• document the expected tradeoffs between imported-graph picking and source-stage-aware picking

Notes

This should stay in StageView so every consumer benefits, with consumer hooks reserved for scene-specific exact mapping.