Add survey data support (Phases 1-2)

TODO.md

@@ -52,6 +52,7 @@ Deferred items from PR reviews that were not addressed before merge.
 | ImputationDiD dense `(A0'A0).toarray()` scales O((U+T+K)^2), OOM risk on large panels | `imputation.py` | #141 | Medium (deferred — only triggers when sparse solver fails; fixing requires sparse least-squares alternatives) |
 | EfficientDiD: warn when cohort share is very small (< 2 units or < 1% of sample) — inverted in Omega*/EIF | `efficient_did_weights.py` | #192 | Low |
 | EfficientDiD: API docs / tutorial page for new public estimator | `docs/` | #192 | Medium |
+| Multi-absorb weighted demeaning needs iterative alternating projections for N > 1 absorbed FE with survey weights; unweighted multi-absorb also uses single-pass (pre-existing, exact only for balanced panels) | `estimators.py` | #218 | Medium |
 | TripleDifference power: `generate_ddd_data` is a fixed 2×2×2 cross-sectional DGP — no multi-period or unbalanced-group support. Add a `generate_ddd_panel_data` for panel DDD power analysis. | `prep_dgp.py`, `power.py` | #208 | Low |
 
 #### Performance

diff_diff/__init__.py

@@ -94,6 +94,10 @@
     PeriodEffect,
     SyntheticDiDResults,
 )
+from diff_diff.survey import (
+    SurveyDesign,
+    SurveyMetadata,
+)
 from diff_diff.staggered import (
     CallawaySantAnna,
     CallawaySantAnnaResults,
@@ -312,6 +316,9 @@
     "compute_pretrends_power",
     "compute_mdv",
     "plot_pretrends_power",
+    # Survey support
+    "SurveyDesign",
+    "SurveyMetadata",
     # Rust backend
     "HAS_RUST_BACKEND",
     # Linear algebra helpers