Fix IndexError in find_blank_cut when the cut lands on the image edge

A cut position after the last column is legitimate — _crop_to_budget asks for
min(start + budget, img.width), which equals the width whenever the remaining
strip is shorter than the budget. find_blank_cut clamped target to width but
then walked leftwards starting at target itself, so ink[width] raised
IndexError.

Caught on hardware: it killed the ledmatrix-stocks fetch, and because
_fetch_plugin_content catches broadly that surfaced as the plugin silently
contributing nothing for the cycle.

Only reachable on the second or later pass of the rotating window over a single
oversized image, which is why the existing tests missed it — they all exercised
the first pass, where start is 0 and start + budget is comfortably inside the
image. Added TestRotationAcrossMultipleCycles, which walks the window round
several times and asserts content is never lost, plus direct coverage of
find_blank_cut at and beyond the image edge.

Both bounds now stop at width - 1 so neither direction can index past the end.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

src/vegas_mode/geometry.py

@@ -222,16 +222,19 @@ def find_blank_cut(
         return target
 
     ink = column_has_ink(img, threshold)
-    lo = max(0, target - search_radius)
-    hi = min(width - 1, target + search_radius)
+
+    # target may legitimately equal width (a cut after the last column), but
+    # there is no column to inspect there, so both bounds stop at width - 1.
+    lo = max(0, min(target - search_radius, width - 1))
+    hi = max(0, min(target + search_radius, width - 1))
 
     # Walk outwards from target so the nearest gap wins.
     for offset in range(0, search_radius + 1):
         right = target + offset
-        if right <= hi and not ink[right]:
+        if lo <= right <= hi and not ink[right]:
             return right
         left = target - offset
-        if left >= lo and not ink[left]:
+        if lo <= left <= hi and not ink[left]:
             return left
 
     return target