Fix did_had_pretest_workflow() docstring: paper Step 4 is the decision rule

Per REGISTRY's pretest workflow (lines 2482-2487 surrounding the
four-step enumeration) and the same module's two correctly-framed
docstrings (module-level at line 54, `_compose_verdict_event_study` at
line 2735), paper Step 4 is the decision rule "use TWFE if none of the
tests rejects" — NOT a test or assumption. The Yatchew-HR test is a
Step 3 alternative (a single-horizon swap-in for Stute), and there is
no joint Yatchew variant in the paper.

The `did_had_pretest_workflow()` docstring at L4445-4446 was the only
place in the file that mislabeled the Yatchew alternative as "Step 4".
Reword to:
- correctly tag Yatchew as a step-3 alternative
- explicitly state Step 4 is the decision rule with no code path
- cross-reference the two existing correctly-framed docstrings in the
  same file so future readers can confirm the convention is consistent

Surfaced by PR #409 R6 review (T21 tutorial pre-test taxonomy work
made the inconsistency visible). All 42 workflow-specific tests in
`tests/test_had_pretests.py` still pass; T21 drift test (16 tests)
still passes.

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

Author: igerber

diff_diff/had_pretests.py

@@ -4442,11 +4442,15 @@ def did_had_pretest_workflow(
 
     ``aggregate="event_study"`` (multi-period panel, >= 3 periods): runs
     QUG + joint pre-trends Stute + joint homogeneity-linearity Stute,
-    covering paper Section 4 steps 1-3 together. Step 4 (Yatchew-style
-    linearity as an alternative to Stute) is subsumed by the joint Stute
-    in this path - the paper does not derive a joint Yatchew variant, so
-    users who need Yatchew robustness under multi-period data should
-    call :func:`yatchew_hr_test` on each (base, post) pair manually.
+    covering paper Section 4 steps 1-3 together. The step-3 Yatchew-HR
+    alternative (a single-horizon swap-in for Stute) is subsumed by joint
+    Stute on this path - the paper does not derive a joint Yatchew
+    variant, so users who need Yatchew robustness under multi-period
+    data should call :func:`yatchew_hr_test` on each ``(base, post)``
+    pair manually. (Paper step 4 is the decision itself - "use TWFE if
+    none of the tests rejects" - not a separate test, so it has no code
+    path here. Mirrors the framing in the module-level docstring at
+    line 54 and ``_compose_verdict_event_study`` at line 2735.)
 
     Eq 17 / Eq 18 linear-trend detrending (paper Section 5.2 Pierce-
     Schott application) is now SHIPPED on the event-study path via