Fix DDD _snap_n floor mismatch in explicit n_range path

Hoist abs_min before the bracket branch so explicit n_range snaps use the
DDD realizable floor (16) instead of the registry profile floor (64).
Also pass floor=abs_min to bisection midpoint snap. Adds regression test
for low-range DDD sample-size search.

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

Author: igerber

diff_diff/power.py

@@ -2316,8 +2316,11 @@ def _power_at_n(n: int) -> float:
         return pwr
 
     # --- Bracket ---
+    abs_min = 16 if is_ddd_grid else 4
     if n_range is not None:
-        lo, hi = _snap_n(n_range[0], "up"), _snap_n(n_range[1], "down")
+        lo, hi = _snap_n(n_range[0], "up", floor=abs_min), _snap_n(
+            n_range[1], "down", floor=abs_min
+        )
         if lo > hi:
             lo = hi  # collapsed bracket — evaluate single point
         power_lo = _power_at_n(lo)
@@ -2351,7 +2354,6 @@ def _power_at_n(n: int) -> float:
         if power_lo >= power:
             # Floor achieves target — search downward for true minimum
             hi = lo
-            abs_min = 16 if is_ddd_grid else 4
             found_lower = False
             probe = _snap_n(max(abs_min, lo // 2), floor=abs_min)
             for _ in range(8):
@@ -2413,7 +2415,7 @@ def _power_at_n(n: int) -> float:
     for _ in range(max_steps):
         if hi - lo <= convergence_threshold:
             break
-        mid = _snap_n((lo + hi) // 2)
+        mid = _snap_n((lo + hi) // 2, floor=abs_min)
         if mid <= lo or mid >= hi:
             break
         pwr = _power_at_n(mid)

tests/test_power.py

@@ -1145,6 +1145,30 @@ def test_ddd_sample_size_grid_aligned(self):
             result.required_n % 8 == 0
         ), f"DDD required_n={result.required_n} is not a multiple of 8"
 
+    @pytest.mark.slow
+    def test_ddd_sample_size_low_range(self):
+        """DDD sample-size search with low n_range stays within bracket."""
+        result = simulate_sample_size(
+            TripleDifference(),
+            n_periods=2,
+            treatment_period=1,
+            treatment_effect=0.5,
+            sigma=5.0,
+            n_simulations=5,
+            n_range=(16, 56),
+            seed=42,
+            progress=False,
+        )
+        assert (
+            result.required_n % 8 == 0
+        ), f"DDD required_n={result.required_n} is not a multiple of 8"
+        assert (
+            16 <= result.required_n <= 56
+        ), f"DDD required_n={result.required_n} outside requested bracket [16, 56]"
+        assert (
+            len(result.search_path) > 2
+        ), f"Bisection should explore >2 points, got {len(result.search_path)}"
+
     @pytest.mark.slow
     def test_trop(self):
         result = simulate_power(