fix(decnet_web/Layout): theme swap animation no longer flashes opposite mode

Growing the NEW theme layer from circle(0) outward leaves a
one-frame gap where the new pseudo is fully opaque at full size
(the default state) before the clip-path animation registers.
Result: a flash of the destination theme right before the
reveal starts.

Inverted the layering and animation direction:
 - NEW theme snapshot sits on the bottom (z-index 0), static
 - OLD theme snapshot sits on top (z-index 1), shrinks via
   clip-path from circle(N) at click point down to circle(0)

The new layer is now hidden behind the old one until the old
shrinks away — no flash possible because the new layer was
never visible before the animation. Same 520ms duration, same
ease curve, same direction-of-travel from the user's POV
(circle expanding from cursor).

decnet_web/src/index.css

@@ -340,18 +340,26 @@ input:focus {
 /* ── Theme transition ───────────────────────────
  * Disables the default cross-fade so the JS-driven
  * circle clip-path in useThemeToggle.ts owns the
- * reveal entirely. The new theme grows from the
+ * reveal entirely.
- * click point; the old theme stays put and is
+ *
- * uncovered by the expanding circle. */
+ * Layering: the NEW theme snapshot sits behind, the
+ * OLD theme snapshot sits on top. The JS animation
+ * shrinks the old layer's clip-path from the
+ * viewport-covering circle down to circle(0) at the
+ * click point — burning away to reveal the new
+ * theme underneath. This avoids the one-frame flash
+ * you'd get by growing the new layer (where the
+ * default fully-opaque pseudo is visible for a tick
+ * before the clip-path animation registers). */
 ::view-transition-old(root),
 ::view-transition-new(root) {
   animation: none;
   mix-blend-mode: normal;
 }
-::view-transition-old(root) {
+::view-transition-new(root) {
   z-index: 0;
 }
-::view-transition-new(root) {
+::view-transition-old(root) {
   z-index: 1;
 }
 

decnet_web/src/lib/useThemeToggle.ts

@@ -70,17 +70,22 @@ function animateSwap(next: Theme, x: number, y: number): void {
       Math.max(x, window.innerWidth - x),
       Math.max(y, window.innerHeight - y),
     );
+    /* Shrink the OLD layer (on top per index.css z-index rules)
+     * away from the click point, uncovering the NEW layer that
+     * sits behind. Going outside-in instead of inside-out avoids
+     * the one-frame flash where the default-opaque new pseudo
+     * would otherwise be visible before the clip-path registers. */
     document.documentElement.animate(
       {
         clipPath: [
-          `circle(0px at ${x}px ${y}px)`,
           `circle(${endRadius}px at ${x}px ${y}px)`,
+          `circle(0px at ${x}px ${y}px)`,
         ],
       },
       {
         duration: 520,
         easing: 'cubic-bezier(0.4, 0, 0.2, 1)',
-        pseudoElement: '::view-transition-new(root)',
+        pseudoElement: '::view-transition-old(root)',
       },
     );
   }).catch(() => { /* user-cancelled or unsupported pseudo, ignore */ });