feat(ribbon): add layer isolation tools

Closes #589

assets/icons/layers/layiso.svg

@@ -0,0 +1,18 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="64" height="64">
+  <!-- Muted surrounding layers -->
+  <line x1="2" y1="5" x2="22" y2="5"
+        stroke="#777777" stroke-width="2.25" stroke-linecap="round"
+        stroke-dasharray="3 3"/>
+  <line x1="2" y1="19" x2="22" y2="19"
+        stroke="#777777" stroke-width="2.25" stroke-linecap="round"
+        stroke-dasharray="3 3"/>
+  <!-- The selected layer remains visible -->
+  <line x1="2" y1="12" x2="22" y2="12"
+        stroke="#4cc9f0" stroke-width="2.25" stroke-linecap="round"/>
+  <circle cx="12" cy="12" r="2.5"
+          fill="#4cc9f0" stroke="#e0e0e0" stroke-width="1.25"/>
+  <!-- Focus brackets -->
+  <path d="M6 9V7H9 M15 7H18V9 M18 15V17H15 M9 17H6V15"
+        fill="none" stroke="#f0c040" stroke-width="1.75"
+        stroke-linecap="round" stroke-linejoin="round"/>
+</svg>

assets/icons/layers/layuniso.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="64" height="64">
+  <!-- Restored layer stack -->
+  <line x1="2" y1="5" x2="22" y2="5"
+        stroke="#e0e0e0" stroke-width="2.25" stroke-linecap="round"/>
+  <line x1="2" y1="12" x2="22" y2="12"
+        stroke="#4cc9f0" stroke-width="2.25" stroke-linecap="round"/>
+  <line x1="2" y1="19" x2="22" y2="19"
+        stroke="#e0e0e0" stroke-width="2.25" stroke-linecap="round"/>
+  <!-- Outward arrows indicate restoring the isolated layers -->
+  <path d="M8 9L5 6L2 9 M16 15L19 18L22 15"
+        fill="none" stroke="#f0c040" stroke-width="1.75"
+        stroke-linecap="round" stroke-linejoin="round"/>
+</svg>