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.

vision.py

@@ -264,6 +264,14 @@ leniently, 75/25 for a 10 and 90/10 from 9 downwards):
   Note that centering is a manufacturing trait, not damage: a badly centred \
   card can still be pristine, and PSA may grade it strongly with an OC \
   (off-centre) qualifier rather than a low number.
+  DIE-CUT and clear-window cards (SPx, E-X Century, jersey- or \
+shape-die-cuts) have no uniform rectangular border, so border-width ratios \
+mostly do not apply to them. Judge centering there as the printed design's \
+registration to the die-cut shape and the card's physical edges — that is \
+what PSA keys off for these issues — and do not reconstruct a border ratio \
+by eye on a card that has no border. If a MEASURED CENTERING block was \
+supplied anyway on such a card, distrust it outright: the measurement \
+assumes a uniform printed border this card does not have.
 - CORNERS: look for whitening, fraying, softness, or blunting at each of the \
 four corners. Sharp corners on all four is 9-10 territory; slight whitening \
 visible under magnification but not to the eye is 8-9; obvious whitening or \
@@ -646,7 +654,20 @@ def grade_card(images, api_key=None, model=None, effort=None, zoom_details=True)
     aspect = cardimage.aspect_profile(images[0][0]) if can_measure else None
 
     prompt_parts = []
-    if centering:
+    if centering and centering.get("reliable") is False:
+        prompt_parts.append(
+            "CENTERING COULD NOT BE MEASURED on this card, because {}. "
+            "You are getting no border-width numbers, and on a card like "
+            "this a border-width ratio is the wrong tool anyway: judge "
+            "centering by eye as the printed design's alignment relative to "
+            "the die-cut shape and the card's physical edges — that is what "
+            "PSA actually keys off for die-cut issues. Do not reconstruct a "
+            "left/right border ratio visually on a card with no uniform "
+            "border; describe what you can genuinely see about the cut's "
+            "registration to the printed pattern, and widen the range if "
+            "the photo doesn't settle it.".format(
+                centering.get("reason", "of its cut")))
+    elif centering:
         prompt_parts.append(
             "MEASURED CENTERING — computed from the pixels by finding the "
             "border on each side of the card:\n"