test: worker correctness tests + hand-crafted engine vs ArroyoSketch comparison

- Add CapturingOutputSink test utility for capturing precomputed outputs in-memory
- Add worker correctness unit tests covering Sum and KLL accumulator pipelines
- Add comparison test asserting precompute engine matches ArroyoSketch results
- Add test that loads aggregation config from real streaming_config YAML
- Expand design doc with sliding window behaviour, accumulator lifecycle,
  load balancing trade-offs, and fault tolerance sections

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zz_y

asap-query-engine/Cargo.toml

@@ -67,6 +67,7 @@ path = "src/bin/precompute_engine.rs"
 name = "test_e2e_precompute"
 path = "src/bin/test_e2e_precompute.rs"
 
+
 [dev-dependencies]
 ctor = "0.2"
 tempfile = "3.20.0"