refactor: eliminate code duplication + add bench_precompute_sketch binary

- Refactor Worker to use WorkerRuntimeConfig struct and Arc<AggregationConfig>
- Add bench_precompute_sketch binary for precompute engine benchmarking
- Add e2e_precompute_equivalence integration test
- Expand design doc with second-tier merge worker design

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

Author: zz_y

asap-query-engine/Cargo.toml

@@ -67,6 +67,9 @@ path = "src/bin/precompute_engine.rs"
 name = "test_e2e_precompute"
 path = "src/bin/test_e2e_precompute.rs"
 
+[[bin]]
+name = "bench_precompute_sketch"
+path = "src/bin/bench_precompute_sketch.rs"
 
 [dev-dependencies]
 ctor = "0.2"