Never crash: Fix blur overflow and hangs on sliver crops

Cap the sigma at twice the picture's long side and square the box width in i64.

docs/whats-new.md

@@ -2,6 +2,10 @@
 
 ## 2026-10-13
 
+### Reliability
+- A crop a hair wide (or of no size), set by an agent or read from a sidecar, no longer crashes or hangs the
+  render with Clarity, Dehaze, Texture or Highlights / Shadows on: their blurs are held to the picture's size.
+
 ### Slider drags on large raws
 - Dragging a slider no longer re-renders the edited photo's filmstrip thumbnail on every frame; it follows once you
   let go. On a 64 MP raw the first of those renders decoded the whole file again, which held up the loupe's