feat(cesium): fill groups A+B scaffold (import/parse + arcgis/decode)

Groups A (tileset, implicit_tiling, khr_gs) and B (arcgis_pbf, spz, esri_crs)
written by Sonnet battery, ///-commented scaffold only, compiling.
- arcgis_pbf grounded verbatim against Esri/arcgis-pbf FeatureCollectionPBuffer
  (field numbers, QuantizeOriginPostion, delta-zigzag dequant) — the no-JSON path.
- spz grounded against nianticlabs/spz (NgspFileHeader 32B LE, stream order,
  dequant formulas; smallest-three quaternion bits marked UNVERIFIED).
- tileset/implicit_tiling/khr_gs grounded against OGC 3D Tiles 1.1 + KHR spec
  (.3tz internals + KHR non-normalized SCALE variants marked UNVERIFIED).
Groups C+D (to_cam_soa/sse/hlod, point_fallback/oracle/fixtures) still stubs;
filled next.

Author: claude