Address PR #409 R7 review (P2 D1) — bounded p-value drift bands

Two bootstrap p-value drift tests had lower-bound-only assertions:
- `test_overall_stute_fails_to_reject`: was `p > 0.50`, tutorial quotes
  ~0.686 → would silently pass if p drifted to 0.99
- `test_event_study_homogeneity_fails_to_reject`: was `p > 0.50`,
  tutorial quotes ~0.763 → same silent-stale risk

The third bootstrap test (`test_event_study_pretrends_fails_to_reject`)
already used a bounded band `0.0 <= p <= 0.25`. Mirror that pattern on
the other two with bounded bands per
`feedback_bootstrap_drift_tests_need_backend_tolerance` (>= 0.15
width):
- Stute: 0.53 <= p <= 0.84 (band ~0.31 around 0.686)
- Homogeneity: 0.61 <= p <= 0.92 (band ~0.31 around 0.763)

Both bands wide enough for Rust ↔ pure-Python RNG path differences;
both narrow enough that drift in either direction (toward rejection
or toward an even cleaner pass) flags the prose as stale.

All 16 drift tests pass on both backends within the new bands.

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

Author: igerber

tests/test_t21_had_pretest_workflow_drift.py

@@ -204,14 +204,14 @@ def test_overall_qug_fails_to_reject(overall_report):
 
 
 def test_overall_stute_fails_to_reject(overall_report):
-    """Section 3 narrative claims Stute fails-to-reject linearity.
-    Stute uses Mammen wild bootstrap so the p-value is RNG-dependent;
-    use binary fail-to-reject + abs tolerance band per
-    `feedback_bootstrap_drift_tests_need_backend_tolerance`."""
+    """Section 3 narrative quotes Stute p_value ~0.686. Stute uses
+    Mammen wild bootstrap so the p-value is RNG-dependent; use a
+    bounded abs tolerance band per
+    `feedback_bootstrap_drift_tests_need_backend_tolerance` (>= 0.15
+    width). Both bounds tight enough to catch methodology drift in
+    either direction, loose enough for backend RNG path differences."""
     assert overall_report.stute.reject is False
-    # Tight enough to catch methodology drift, loose enough for backend
-    # RNG path differences.
-    assert overall_report.stute.p_value > 0.50, overall_report.stute.p_value
+    assert 0.53 <= overall_report.stute.p_value <= 0.84, overall_report.stute.p_value
 
 
 def test_overall_yatchew_fails_to_reject(overall_report):
@@ -292,11 +292,16 @@ def test_event_study_pretrends_fails_to_reject(event_study_report):
 
 def test_event_study_homogeneity_fails_to_reject(event_study_report):
     """Section 4 narrative claims joint homogeneity strongly fails to
-    reject (~0.76 from numbers.json)."""
+    reject and quotes p ~0.763 from numbers.json. Use a bounded abs
+    tolerance band per
+    `feedback_bootstrap_drift_tests_need_backend_tolerance` so that
+    drift in either direction (toward rejection or toward an even
+    cleaner pass) flags the prose as stale rather than silently
+    passing."""
     hj = event_study_report.homogeneity_joint
     assert hj is not None
     assert hj.reject is False
-    assert hj.p_value > 0.50, hj.p_value
+    assert 0.61 <= hj.p_value <= 0.92, hj.p_value
 
 
 def test_had_design_auto_lands_on_continuous_at_zero(two_period):