Layers panel: ⌥-click on the line between layers, measured against Photoshop (#1642)

#1066 added the gesture; this makes it behave as Photoshop 25.4 does (measured by sweeping
⌥-hover and ⌥-click a pixel at a time across each line, results read back by script):

- the line takes the pointer from 3 px above it to 5 px below it (was ±3);
- a release frees the layer above the line together with the clipped layers stacked above it,
  in one history step, and makes it the active layer (was: that layer alone); a clip keeps the
  active layer, as before;
- clipping and releasing have their own cursors: PhotoCraft's original clip-below (a hooked
  arrow onto the layer below) and clip-release (the same arrow with a small "no" badge),
  replacing Lucide corner-right-down for both.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

assets/icons/clip-below.svg

@@ -0,0 +1,16 @@
+<svg
+  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"
+>
+  <!-- PhotoCraft's own clipping-mask cursor (not from an icon set): a hooked arrow down onto the layer below. -->
+  <path d="M5 4h4.5a5.5 5.5 0 0 1 5.5 5.5V16" />
+  <path d="m11.5 12.5 3.5 3.5 3.5-3.5" />
+  <path d="M5 20.5h14" />
+</svg>

assets/icons/clip-release.svg

@@ -0,0 +1,18 @@
+<svg
+  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"
+>
+  <!-- PhotoCraft's own release-clipping-mask cursor: clip-below.svg unchanged, plus a small "no" badge (thinner strokes keep it legible at cursor size). -->
+  <path d="M5 4h4.5a5.5 5.5 0 0 1 5.5 5.5V16" />
+  <path d="m11.5 12.5 3.5 3.5 3.5-3.5" />
+  <path d="M5 20.5h14" />
+  <circle cx="20" cy="4" r="3.25" stroke-width="1.5" />
+  <path d="m17.7 1.7 4.6 4.6" stroke-width="1.5" />
+</svg>

assets/icons/corner-right-down.svg

@@ -1,14 +0,0 @@
-<svg
-  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>