Add --min-why threshold to filter Stage 3 selections by confidence level

Add diagnostic eval metrics, why-distribution tracking, and generic character filter

Add parallel processing to eval pipeline with ThreadPoolExecutor

Add independent character tag metrics to eval pipeline

Improve eval harness: shuffle samples, always write results

Add end-to-end evaluation harness for pipeline metrics

Expand alias filter tests with real CSV data and pipeline tests

Add alias-based character tag filtering for Stage 3