feat: bucket selection matching Klein 9B table

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_buckets.py

@@ -0,0 +1,27 @@
+from gates import buckets
+
+# (iw, ih) -> expected (W, H) from KLEIN_BUCKET_SIZES.md, budget 1280, ÷64
+CASES = [
+    (1000, 1000, 1280, 1280),   # square
+    (1000, 2000, 896, 1792),    # a=0.50 portrait
+    (1000, 1730, 960, 1664),    # a≈0.58
+    (1000, 1100, 1216, 1344),   # a≈0.90 -> portrait-leaning
+    (2000, 1000, 1792, 896),    # a=2.00 landscape
+    (1500, 1000, 1536, 1024),   # a=1.50
+]
+
+
+def test_pick_bucket_matches_table():
+    for iw, ih, W, H in CASES:
+        assert buckets.pick_bucket(iw, ih, 1280, 64) == (W, H)
+
+
+def test_buckets_are_on_grid_and_within_budget():
+    for iw, ih, *_ in CASES:
+        W, H = buckets.pick_bucket(iw, ih, 1280, 64)
+        assert W % 64 == 0 and H % 64 == 0
+        assert W * H <= 1280 * 1280
+
+
+def test_square_is_exactly_1280():
+    assert buckets.pick_bucket(512, 512, 1280, 64) == (1280, 1280)