Implement structural memory system for REMESH with centralized coherence

Adds full structural field memory implementation as specified in issue #2670:
- compute_structural_signature: Multidimensional signatures (EPI, νf, θ, ΔNFR, topology)
- detect_recursive_patterns: Pattern clustering with similarity thresholds
- identify_pattern_origin: Select strongest pattern instance (EPI × νf)
- propagate_structural_identity: Interpolated identity propagation
- apply_network_remesh_with_memory: Full integration with standard REMESH

Key improvements:
- Eliminated redundancy: Uses centralized compute_coherence() from tnfr.metrics
- 70 tests passing (40 existing + 30 new)
- Preserves all TNFR canonical invariants
- Phase verification for coupling (U3)
- Structural bounds preservation via structural_clip

Co-authored-by: fermga <203334638+fermga@users.noreply.github.com>

Author: Copilot