Address PR #353 CI review round 6 (1 P3)

P3 - stute_joint_pretest docstring drift:
The Raises block still listed `G < _MIN_G_STUTE` as a ValueError
condition, but R5 converted that branch to a UserWarning + full-NaN
StuteJointResult return to match single-horizon stute_test and
keep the event-study workflow from crashing on staggered-filtered
small panels.

Fix: rewrote the Returns and Raises docstring blocks to describe
the actual contract. Returns now enumerates the three NaN-result
branches (small G, constant dose, any-NaN residuals / fitted) with
their warning behavior. Raises is narrowed to the genuinely-raising
conditions: empty input, key-mismatch, str-label collision, shape
mismatch, negative doses, too-few bootstrap replicates, invalid
alpha. Explicitly notes that small-G does NOT raise.

No code changes; docstring-only edit. 126 tests still pass;
black/ruff/mypy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/had_pretests.py

@@ -1961,13 +1961,25 @@ def stute_joint_pretest(
     Returns
     -------
     StuteJointResult
+        On the common path, a populated result with bootstrap-based
+        ``p_value`` and ``cvm_stat_joint``. On the small-sample branch
+        (``G < _MIN_G_STUTE``), constant-dose branch
+        (``np.ptp(doses) <= 0``), or any-NaN branch in the input
+        residuals / fitted arrays, returns an all-NaN result (with
+        ``reject=False`` and the full ``per_horizon_stats`` dict keyed
+        by the validated horizon labels) and emits a ``UserWarning``
+        for the first two branches. Mirrors the single-horizon
+        :func:`stute_test` contract so event-study workflows on small
+        or staggered-filtered panels surface an inconclusive report
+        rather than crashing.
 
     Raises
     ------
     ValueError
-        On empty input, key-mismatch, shape-mismatch, ``doses``
-        containing negative values, ``G < _MIN_G_STUTE``, or
-        ``n_bootstrap < _MIN_N_BOOTSTRAP``.
+        On empty input, key-mismatch, stringified-label collisions
+        between distinct raw keys, shape-mismatch, ``doses`` containing
+        negative values, ``n_bootstrap < _MIN_N_BOOTSTRAP``, or invalid
+        ``alpha``. ``G < _MIN_G_STUTE`` does NOT raise; see Returns.
     """
     if not isinstance(residuals_by_horizon, dict) or not isinstance(fitted_by_horizon, dict):
         raise ValueError(