Layers panel: Alt-click the line between two layers clips the upper one (#967) (#1066)

Holding Alt (Option) over the line between two layers in the Layers panel
shows the clip cursor; a click runs Layer > Create Clipping Mask on the upper
layer, or Release Clipping Mask when it is clipped already, as in Photoshop.
The line only counts between siblings that are next to each other in the
stack (not inside an open group or across filtered-out rows), and stays
clear of the eye column and the effects triangle. The cursor is the Lucide
corner-right-down icon.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

assets/icons/corner-right-down.svg

@@ -0,0 +1,14 @@
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+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="m10 15 5 5 5-5" />
+  <path d="M4 4h7a4 4 0 0 1 4 4v12" />
+</svg>