Refactored PromptBuilder to support both legacy (sync) and new (async) optimization paths: Legacy (backward compatible): - cache_metrics() still uses sync ContextOptimizer - build_cache_aligned() unchanged, no optimization New (pluggable OptimizerService): - cache_metrics() falls back gracefully to ContextOptimizer - NEW: build_cache_aligned_async() uses pluggable service - Custom optimizers now work in prompt building Architecture Benefits: ✅ Generic registry optimization works everywhere (ingest + query) ✅ Same codebase supports multiple compressors ✅ Async-aware for production query paths ✅ Backward compatible (no breaking changes) Usage in query_executor: Tests: All 14 prompt tests passing (no changes to test surface)