Portable continuity gate

One command:
  python3 continuity_gate.py fixture_pass.json

Expected: PASS. The negative control must fail:
  python3 continuity_gate.py fixture_fail.json

The gate rejects unknown contract, INHERITED provenance, non-contiguous IDs, and expected-head mismatch. It is fixture-level evidence, not adoption or proof against a live session ledger.