Address P3: MultiPeriodDiD wild bootstrap warns, not errors

Split wild bootstrap row: DiD/TWFE raise NotImplementedError,
MultiPeriodDiD warns and falls back to analytical inference.

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

Author: igerber

docs/survey-roadmap.md

@@ -327,7 +327,8 @@ Combining them raises `NotImplementedError` or `ValueError`:
 | SyntheticDiD | `variance_method='placebo'` + strata/PSU/FPC | Use `variance_method='bootstrap'` |
 | ImputationDiD | `pretrends=True` + replicate weights | Per-replicate lead regression refits not implemented |
 | ImputationDiD | `pretrend_test()` + replicate weights | Per-replicate Equation 9 refits not implemented |
-| DifferenceInDifferences, TwoWayFixedEffects, MultiPeriodDiD | `inference='wild_bootstrap'` + `survey_design` | Use analytical survey inference (the default) instead |
+| DifferenceInDifferences, TwoWayFixedEffects | `inference='wild_bootstrap'` + `survey_design` | Raises `NotImplementedError`; use analytical survey inference (the default) instead |
+| MultiPeriodDiD | `inference='wild_bootstrap'` + `survey_design` | Warns and falls back to analytical inference (no error raised) |
 
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