fix(weights): tighten observability and edge-case handling in label pipeline

fix(weights): tighten observability and edge-case handling in label pipeline

- sanitize_and_renormalize accepts logger/context kwargs and warns on
  uniform-fallback collapse; six call sites in QuickAdapterRegressorV3
  thread their stage label (train_test_split / post_feature_pipeline /
  timeseries_split, train|test).
- Warn at startup when label_prediction.method='none' for any label, since
  populate_entry_trend would silently never trigger.
- Replace .notna() with np.isfinite() in the smoothed-weight clip so +Inf
  produced by smoothing kernels is also zeroed instead of relying on the
  downstream drop_mask in compose_sample_weights.
- _impute_weights tracks boundary NaN separately so injected zeros do not
  bias the interior median; finite endpoints are now preserved.