feat(3d-tiles): add traversal observability and benchmark baselines - #4018

Goals

• Make 3D Tiles traversal decisions reproducible for conformance tests and performance investigations.
• Expose request, cache, memory, and implicit-subtree diagnostics without coupling the loader runtime to a renderer.
• Document benchmark dimensions and practical budgets for future Cesium-parity work.

Changes

• Add public Tileset3DTraversalSnapshot and getTileset3DTraversalSnapshot to @loaders.gl/tiles.
• Expose defensive requestedTiles and emptyTiles views on Tileset3D.
• Capture sorted selected/requested/empty IDs, renderable counts, loading/failure/cache counters, resident bytes, and active SSE.
• Include implicit-subtree request, cache-hit, pending, and materialization counters when supplied by Tiles3DSource.
• Add a 3D Tiles observability and benchmark guide, with links from the module and Tileset3D API pages.
• Add hermetic Vitest coverage for deterministic ordering, defensive snapshots, and source diagnostics.
• Add a v5 What's New bullet.

Tracker and scope

Partially addresses #1245, R3 (observability and benchmarks). This tranche is additive and does not change traversal policy, styling, GPU upload, or renderer draw decisions. R2 styling/feature access remains dependent on the existing R1 contracts and metadata work.

Validation

CI will run lint, builds, Node/headless tests, coverage, and the website build.