test(engines): add comprehensive unit and property-based tests for core engines

[NP-compete]

Summary

Adds comprehensive unit tests and property-based tests (Hypothesis) for all core engines, raising engine test coverage from 81% to 96%.

• Property-based tests using Hypothesis verify mathematical invariants:
  • Decay is monotonically decreasing without rehearsal
  • Strength always bounded by [min_strength, 1.0]
  • Pinned memories never decay
  • Importance always in [0, 1]
  • Frequency/emotional scores are monotonic w.r.t. inputs
• Unit test gap coverage for from_config, apply_rehearsal_in_place, naive datetime handling, invalid metadata, edge cases in threshold estimation, and MMR diversity

Coverage Improvement

Engine	Before	After
decay.py	82%	95%
importance.py	92%	100%
retrieval.py	94%	97%
consolidation.py	95%	95%
working_memory.py	35%*	95%
TOTAL	81%	96%

* working_memory was undercounted (test file excluded from prior run)

Files Changed

File	Change
tests/property/__init__.py	New property test package
tests/property/test_decay_properties.py	6 property tests (~1,070 examples)
tests/property/test_importance_properties.py	7 property tests (~970 examples)
tests/unit/test_decay.py	+14 tests (from_config, rehearsal, naive dt, edge cases)
tests/unit/test_importance.py	+10 tests (from_config, naive dt, invalid metadata)
tests/unit/test_retrieval.py	+8 tests (from_config, edges, naive dt, MMR)

Test plan

• 338 total tests pass (176 engine-specific)
• Ruff lint + format clean
• Property tests verify invariants across randomized inputs
• All from_config class methods tested
• Naive datetime handling tested for all engines
• Edge cases tested (zero strength, threshold boundaries, empty inputs)

Closes #14