refactor(weights): tighten label-weight pipeline contract

refactor(weights): tighten label-weight pipeline contract

- compute_label_weights raises on strategy='none'; callers must skip
- _scatter_weights default_weight is required (no implicit baseline)
- gate weight-column writes on is_weighting_active (strategy AND pivots)
- demote 'label weight column absent' log to debug (legitimate path)
- remove DEFAULT_LABEL_WEIGHT (conflated three unrelated 1.0 semantics)
- harden sanitize_and_renormalize fallback when drop_mask covers all rows
- add Final[str] / Final[re.Pattern[str]] for type coherence