feat: implement canonical witness fragments, tests, and lib.rs wiring

- cognitum-gate-kernel: canonical_witness module with no_std ArenaCactus,
  FixedPointWeight, CanonicalPartition (bitset-based), CanonicalWitnessFragment,
  FNV-1a hashing, BFS spanning tree for cactus construction (912 lines)
- ruvector-mincut: canonical tests for determinism, correctness, fixed-weight
  ordering, cactus construction, witness receipts (548 lines)
- ruvector-mincut: wire canonical module into lib.rs with feature-gated
  re-exports and prelude additions
- ruvector-coherence: spectral module refinements

https://claude.ai/code/session_018QKTLyCUrMUQCRDqoiyEHY

Author: claude