StatsPAI 0.7.1 — DID polish release

DID-focused polish release. Brings the Wooldridge (2021) ETWFE
implementation to full feature parity with the R etwfe package,
adds a one-call method-robustness workflow, and closes 12 issues
uncovered by an internal code review round. All 27 new / updated
DID tests pass (pytest tests/test_did_summary.py).

Added — ETWFE full parity with R etwfe

• sp.etwfe() explicit API aligned with R etwfe (McDermott 2023)
  naming. Thin alias over wooldridge_did() with a full argument-
  mapping table in the docstring.
• xvar= covariate heterogeneity (single string or list of names).
  Adds per-cohort × post × (x_j − mean(x_j)) interactions; detail
  gains slope_<x> / slope_<x>_se / slope_<x>_pvalue columns.
  Baseline ATT is reported at the sample means of every covariate.
• panel=False repeated cross-section mode — replaces unit FE
  with cohort + time dummies (R etwfe(ivar=NULL) equivalent).
• cgroup='nevertreated' — per-cohort regressions restricted to
  (cohort g) ∪ (never-treated); cohort-size-weighted aggregation
  (R etwfe(cgroup='never') equivalent). Default 'notyet' preserves
  prior ETWFE behaviour.
• sp.etwfe_emfx(result, type=…) — R etwfe::emfx-equivalent
  four aggregations: 'simple', 'group', 'event', 'calendar'.
  include_leads=True returns full event-time output including pre-
  treatment leads for pre-trend inspection (rel_time = -1 is the
  reference category).

Added — one-call DID method-robustness workflow

• sp.did_summary() — fits five modern staggered-DID estimators
  (CS, SA, BJS, ETWFE, Stacked) to the same data and returns a tidy
  comparison table with per-method (estimate, SE, p, 95 % CI). Mean
  across methods + cross-method SD flag method-sensitivity of results.
• include_sensitivity=True — attaches the Rambachan-Roth (2023)
  breakdown M* to the CS row, giving a three-way robustness readout
  in a single call.
• sp.did_summary_plot() — forest plot of per-method estimates
  with cross-method mean line; sort_by='estimate' supported.
• sp.did_summary_to_markdown() / _to_latex() — publication-
  ready exports (GFM tables / booktabs LaTeX with auto-escaped
  ampersands).
• sp.did_report(save_to=dir) — one-call bundle that writes
  did_summary.txt / .md / .tex / .png / .json to a folder.

Fixed — 12 issues from the internal code review

Blockers (C-severity):

• etwfe(xvar=…) now raises a clear ValueError when the covariate
  is all-NaN or (near-)constant. Previously returned n_obs = 0, estimate = 0 silently.
• etwfe(panel=False, cgroup='nevertreated') now raises a crisp
  NotImplementedError instead of silently falling back to
  'notyet'.
• did_summary now validates column names up front (raises
  KeyError listing missing columns) and only catches narrow
  estimator-side exceptions inside the fit loop; user typos in
  controls= / cluster= surface as proper errors.
• did_summary results round-trip cleanly through stdlib
  serialisation (DIDSummaryResult(CausalResult) subclass with a
  real .summary() method, replacing the prior closure-bound
  instance attribute).

High-severity:

• etwfe_emfx(type='event'/'calendar') now computes SEs via the
  delta method on the stored event-study vcov instead of the
  independent-coefficient approximation. model_info['se_method']
  advertises which path was used.
• etwfe_emfx(type='group') headline se / pvalue / ci are now
  populated (match the underlying fit's overall ATT exactly).
• Validation for did_summary_plot / _to_markdown / _to_latex
  aligned on a single sentinel model_info['_did_summary_marker'].
• _etwfe_never_only no longer leaves a _ft_cache helper column
  on the caller's DataFrame.
• Slope indexing in _etwfe_with_xvar is now name-keyed
  (coef_index dict); regression test verifies swapping
  xvar=['x1','x2'] vs ['x2','x1'] produces identical slopes per
  name.
• etwfe(panel=False) with rank-deficient designs emits a
  RuntimeWarning pointing at concrete remedies (previously fell
  through to pinv silently).

Tests

• New test module tests/test_did_summary.py — 27 cases covering
  consistency with direct estimator calls, export formats, forest
  plot rendering, etwfe_emfx round-trips, xvar / panel / cgroup
  options, the 12 review fixes, and the include_leads mode.