Add distribution diagnostics for BalanceDF (#265)

Summary:
- Added weighted EMD/CVMD/KS computation helpers and comparison functions in the weighted stats module.
- Exposed EMD/CVMD/KS BalanceDF helper methods and public comparison APIs for linked samples and direct targets.
- Added appropiate tests for EMD/CVMD/KS covering identical distributions, weighted effects, expected discrete/numeric values, validation errors, and NA-indicator skipping.

Pull Request resolved: #265

Differential Revision: D90854392

Pulled By: talgalili

fbshipit-source-id: 6a29d41d960131b58ade6ee139de0466fc8d0b0d

Author: neuralsorcerer

CHANGELOG.md

@@ -27,6 +27,9 @@
   - `descriptive_stats()` now accepts a `formula` argument that is always
     applied to the data (including numeric-only frames), letting callers
     control which terms and dummy variables are included in summary statistics.
+- **Added EMD/CVMD/KS distribution diagnostics**
+  - `BalanceDF` now exposes Earth Mover's Distance (EMD), Cramér-von Mises distance (CVMD), and Kolmogorov-Smirnov (KS) statistics for comparing adjusted samples to targets.
+  - These diagnostics support weighted or unweighted comparisons, apply discrete/continuous formulations, and respect `aggregate_by_main_covar` for one-hot categorical aggregation.
 
 ## Bug Fixes