Add comprehensive U5 test suite and fix existing tests

Intent: Complete test coverage for U5 Multi-Scale Coherence rule
Operators involved: REMESH (Recursivity), IL (Coherence), THOL (Self-organization)
Affected invariants: #7 (Operational Fractality)

Key changes:
- Created test_u5_multiscale_coherence.py with 25 comprehensive tests
- Test operator sets, shallow/deep recursion, stabilizer windows, edge cases
- Test U5 independence from U2+U4b (decisive test case)
- Test U5 integration with complete grammar pipeline
- Fixed existing test expecting 7 messages (now 8 with U5)
- Fixed THOL test to include U4b context (destabilizer)

Test coverage:
- Shallow recursion (depth=1): passes without stabilizer ✓
- Deep recursion (depth>1): fails without stabilizer ✓
- Deep recursion with stabilizers: passes ✓
- Stabilizer window (±3 operators): enforced ✓
- Independence from U2+U4b: proven ✓
- Integration with all rules: verified ✓

All 93 grammar tests pass (68 existing + 25 new U5 tests)

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

Author: Copilot