fix: address CI review round 10 - _snap_n floor enforcement for n_range

- P1: fix _snap_n() to honor floor even when grid_step==1 (non-DDD),
  so explicit n_range with survey_config is clamped to min_viable_n
- Add regression test for n_range=(10, 200) with min_viable_n=80

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/power.py

@@ -2834,9 +2834,9 @@ def simulate_sample_size(
         )
 
     def _snap_n(n: int, direction: str = "down", floor: Optional[int] = None) -> int:
-        if grid_step == 1:
-            return n
         actual_floor = floor if floor is not None else min_n
+        if grid_step == 1:
+            return max(actual_floor, n)
         if direction == "up":
             return max(actual_floor, ((n + grid_step - 1) // grid_step) * grid_step)
         return max(actual_floor, (n // grid_step) * grid_step)

tests/test_power.py

@@ -2395,6 +2395,25 @@ def test_survey_sample_size_large_floor_auto_bracket(self):
         )
         assert result.required_n >= cfg.min_viable_n  # must be >= 200
 
+    def test_survey_sample_size_explicit_n_range_clamped(self):
+        """Explicit n_range below survey floor is clamped to min_viable_n."""
+        cfg = SurveyPowerConfig(n_strata=5, psu_per_stratum=8)
+        # n_range=(10, 200) but min_viable_n=80, so lo should be clamped to 80
+        result = simulate_sample_size(
+            CallawaySantAnna(),
+            treatment_effect=3.0,
+            sigma=1.0,
+            n_range=(10, 200),
+            n_simulations=10,
+            max_steps=3,
+            seed=42,
+            survey_config=cfg,
+            n_periods=4,
+            treatment_period=2,
+            progress=False,
+        )
+        assert result.required_n >= cfg.min_viable_n  # must be >= 80
+
     def test_survey_rejects_heterogeneous_te(self):
         """heterogeneous_te_by_strata=True rejected with simulation power."""
         cfg = SurveyPowerConfig(heterogeneous_te_by_strata=True)