feat: bucket selection matching Klein 9B table

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

gates/buckets.py

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+"""Pure bucket math for KLEIN_BUCKET_SIZES.md. Stdlib only."""
+import math
+
+
+def pick_bucket(iw, ih, resolution=1280, divisible=64):
+    """Choose the on-grid bucket (W,H), area <= resolution^2, nearest to the
+    image aspect (log distance; tie-break larger area)."""
+    budget = resolution * resolution
+    target = iw / ih
+    best = None
+    w = divisible
+    w_max = budget // divisible
+    while w <= w_max:
+        h = (budget // w // divisible) * divisible      # largest on-grid h within budget
+        if h >= divisible:
+            err = abs(math.log(w / h) - math.log(target))
+            cand = (err, -(w * h), w, h)                 # min err, then max area
+            if best is None or cand < best:
+                best = cand
+        w += divisible
+    return best[2], best[3]
+
+
+def cover_crop_params(iw, ih, W, H):
+    """Cover-scale + centered crop to land (iw,ih) exactly on (W,H)."""
+    scale = max(W / iw, H / ih)
+    new_w = max(W, round(iw * scale))
+    new_h = max(H, round(ih * scale))
+    left = (new_w - W) // 2
+    top = (new_h - H) // 2
+    return new_w, new_h, left, top, scale