Refuse centering measurement on die-cut/clear-window cards

Same failure family as the edge-whitening fix: centering_profile assumed a
uniform printed border, so on an E-X/SPx-style die-cut the side midpoint
samples landed on clear acetate and the border-width walk measured two
different materials, producing a confident 70/30 the model then treated as
authoritative. Now the four side samples must read as the same material or
the measurement refuses with the reason; the prompt tells the model to judge
die-cut centering as design-to-cut registration instead.

cardimage.py

@@ -619,6 +619,16 @@ def centering_profile(image_bytes):
     Returns None when the card has no uniform border to measure against
     (full-bleed modern cards) or the read looks implausible, so the caller
     can fall back to the model's own eye rather than trust a bad number.
+
+    Returns {"reliable": False, "reason": ...} — a refusal with an
+    explanation, distinct from a silent None — when the four sides read as
+    DIFFERENT materials from each other. A die-cut card with a clear window
+    (SPx, E-X Century) is the case that forced this: the side midpoints land
+    on acetate showing the background through it, the walk inward measures
+    the widths of two different materials, and the result is a confident
+    70/30 on a card whose ratio was never measurable — which the model then
+    treats as authoritative and caps the grade with. Same failure family as
+    the per-edge outlier check in edge_wear_profile, one measurement over.
     """
     if Image is None:
         return None
@@ -633,10 +643,51 @@ def centering_profile(image_bytes):
         px = card.load()
 
         # The border colour, sampled just inside the cut at the midpoint of
-        # each side — far from corners and from any design element.
+        # each side — far from corners and from any design element. A small
+        # patch mean per side rather than one pixel, since a single pixel of
+        # noise shouldn't decide whether the whole measurement runs.
         inset = max(2, int(min(w, h) * 0.012))
-        samples = [px[inset, h // 2], px[w - 1 - inset, h // 2],
-                   px[w // 2, inset], px[w // 2, h - 1 - inset]]
+        patch = max(2, int(min(w, h) * 0.008))
+
+        def patch_mean(cx, cy):
+            total = [0, 0, 0]
+            n = 0
+            for x in range(max(0, cx - patch), min(w, cx + patch + 1)):
+                for y in range(max(0, cy - patch), min(h, cy + patch + 1)):
+                    p = px[x, y]
+                    total[0] += p[0]; total[1] += p[1]; total[2] += p[2]
+                    n += 1
+            return (total[0] // n, total[1] // n, total[2] // n)
+
+        side_samples = {
+            "left": patch_mean(inset, h // 2),
+            "right": patch_mean(w - 1 - inset, h // 2),
+            "top": patch_mean(w // 2, inset),
+            "bottom": patch_mean(w // 2, h - 1 - inset),
+        }
+
+        # All four sides must plausibly be the SAME border before a ratio of
+        # their widths means anything. A side whose colour sits far from the
+        # median of the other three is a different material — a clear die-cut
+        # window showing the background, a foil accent strip — and one such
+        # side invalidates the whole geometric premise, not just its own
+        # number: the walk inward on every side keys off one shared
+        # border_rgb. Refuse with the reason rather than return a ratio.
+        for side, colour in side_samples.items():
+            others = [v for s, v in side_samples.items() if s != side]
+            med = tuple(sorted(o[i] for o in others)[len(others) // 2]
+                        for i in range(3))
+            if sum(abs(colour[i] - med[i]) for i in range(3)) > 150:
+                return {
+                    "reliable": False,
+                    "reason": ("the {} side's border reads as a completely "
+                               "different colour/material from the other "
+                               "sides — typical of a die-cut or clear-window "
+                               "card, where a border-width ratio doesn't "
+                               "describe centering at all").format(side),
+                }
+
+        samples = list(side_samples.values())
         border_rgb = tuple(sorted(c[i] for c in samples)[len(samples) // 2]
                            for i in range(3))
 
@@ -668,6 +719,7 @@ def centering_profile(image_bytes):
             return None
 
         return {
+            "reliable": True,
             "widths_px": widths,
             "horizontal": horizontal,
             "vertical": vertical,