Implement StrategyCoordinator and FusionEngine as named facades over
the existing ACMS pipeline components, providing clean public APIs
for parallel strategy execution with proportional budget allocation
and fragment fusion with dedup/conflict resolution/knapsack packing.
Key changes:
- Add StrategyCoordinator with parallel execution and confidence-based budget allocation
- Add FusionEngine with URI+hash dedup, max-depth conflict resolution, greedy knapsack packing
- Add budget overage guard with lowest-relevance fragment dropping
- Add per-strategy max caps enforcement
- Wire into existing ContextAssemblyPipeline
- Add Behave BDD tests, Robot integration tests, ASV benchmarks
- Add docs/reference/acms_fusion.md
ISSUES CLOSED: #192
