feat(cesium): implement tileset.rs cold-import parser (no-serde, Group-A entry)

Fill the commented tileset.rs scaffold with a live OGC 3D Tiles 1.1
parser per the crate's RULE 3 (no serde, no json crates): a dependency-
free hand-rolled tokeniser parses at the cold boundary into a transient
JSON model that is walked into owned structs and dropped before return —
no JSON value reaches the hot path.

- Owned model: Tileset / TilesetAsset / TileNode / enum BoundingVolume /
  Refine / TileContent / ImplicitTilingRef / SubdivisionScheme / ParseError.
- content+contents unified into one Vec; refine inheritance resolved at
  traversal; uri/legacy-url accepted; box/region/sphere dispatch.
- 10 unit tests; 67/67 crate tests pass; tileset.rs clippy-clean.

Fills the entry point of the Group-A ingest front-end (khr_gs.rs and
implicit_tiling.rs remain scaffold). Refactored from the audit session's
reader logic onto cesium's owned-struct contract. Proposed for
Opus + CodeRabbit review per the crate's scaffold contract.

https://claude.ai/code/session_01UHfLwSzGfVNzkNrTEBijuA

Author: claude