feat(signal): ADR-142 evolution tracker + temporal VoxelMap (#846)

- ruvsense/evolution.rs (extends ADR-030):
  - TemporalVoxel: Bayesian log-odds occupancy update, evidence_count,
    confidence = 1-exp(-count/5) (5-frame low-confidence floor), Welford
    variance, doppler attribution, last_update_ns
  - TemporalVoxelMap: persistent grid, observe(), low_confidence_indices()
  - EvolutionTracker: per-link Welford baselines + cross-link change-point
    (>=3 links beyond 2sigma in one window); divergence checked vs prior baseline
  - VoxelGate: privacy demotion (Anonymous clears doppler+confidence, keeps
    occupancy; Restricted → occupancy histogram only, raw map cleared)
- reuses field_model::WelfordStats; 6 tests; workspace 0 errors

Co-Authored-By: claude-flow <ruv@ruv.net>