Datasets:
Tighten verification and relationship documentation
Consolidates archive semantics, adds structural relationship and physical-reference validators with tests, binds physical-reference verification to pinned source receipts, and reduces stale explanatory clutter. This PR changes 14 documentation, sidecar, and validation files only; it does not change canonical media or active metadata tables.
Validated exact PR head 8be4c3a9cc9dccdc73149f171e527f493e312ce0: 14 intended paths only; zero media or active metadata changes; 537 MP4 identities unchanged; 532 active rows pass exact-head inventory validation; 18 tests pass; both fragment sidecars pass structural and exact historical-revision validation; physical-reference validation passes. The Dataset Viewer API serves main rather than PR refs, so exact-head checks used immutable Hub and Git objects plus remote-byte receipts. Left unmerged.
Merged after exact remote-head validation at 8be4c3a9cc9dccdc73149f171e527f493e312ce0. Scope remained 14 intended documentation, relationship, and validation paths with no media or active metadata changes.