fix(canvas): las placas solo pintan encima cuando estan POSADAS en un zocalo

El bump global a zIndex 3 (efecto iman del Round Display) puso TODAS las
placas por encima de TODOS los componentes: una resistencia al lado de un
Arduino quedaba oculta tras la placa en cualquier ejemplo normal (se veian
los cables, no el cuerpo). Ahora isBoardSeated (misma matematica del snap,
umbral 0.5px) decide: posada -> z3 encima de su zocalo, como el XIAO fisico
apilado en el shield; libre -> z0 debajo de los componentes, como siempre.
El iman coloca la placa exactamente en el asiento, asi que al capturarla
salta al frente y al arrancarla vuelve abajo.

frontend/src/components/simulator/BoardOnCanvas.tsx

@@ -1,4 +1,6 @@
 import React from 'react';
+import { useSimulatorStore } from '../../store/useSimulatorStore';
+import { isBoardSeated } from '../../utils/socketSnap';
 import { getProBoard } from '../../lib/proBoardRegistry';
 import type { BoardInstance } from '../../types/board';
 import { ArduinoUno } from '../velxio-components/ArduinoUno';
@@ -103,6 +105,14 @@ export const BoardOnCanvas = ({
 }: BoardOnCanvasProps) => {
   const { id, boardKind, x, y } = board;
   const size = BOARD_SIZE[boardKind] ?? getProBoard(boardKind)?.size ?? { w: 300, h: 200 };
+  // Seated on a socket component (Round Display back header)? Decides the
+  // stacking below. Recomputed on every position change; the check is a few
+  // pad lookups over the component list, cheap at render rate.
+  const components = useSimulatorStore((st) => st.components);
+  const seated = React.useMemo(
+    () => isBoardSeated(id, boardKind, x, y, components),
+    [id, boardKind, x, y, components],
+  );
 
   // Status dot color: green=running, amber=compiled, gray=idle
   const statusColor = board.running ? '#22c55e' : board.compiledProgram ? '#f59e0b' : '#6b7280';
@@ -184,12 +194,15 @@ export const BoardOnCanvas = ({
     // sibling of PinOverlay) made moving onto a pin fire mouseleave, which
     // cleared the hover and hid the pins before you could click one.
     <div
-      // Boards paint ABOVE components (which sit at zIndex 1, 2 selected).
-      // The case that decides it is stacking: a XIAO seated on the Round
-      // Display's socket must be the thing you see, the way the board you
-      // just dropped is the thing on top of the pile. Side-by-side layouts
-      // never overlap, so nothing else changes.
-      style={{ position: 'absolute', left: 0, top: 0, zIndex: 3 }}
+      // Stacking: boards normally sit BELOW components (z 0 vs their 1/2) —
+      // a resistor next to an Arduino must be visible on top, and a blanket
+      // z bump here once hid it behind the board in every ordinary example.
+      // The ONE exception is a board SEATED on a socket component (the Round
+      // Display back header): then the board is the thing you see, the way
+      // the physical XIAO stacks on the shield. The magnet snap places the
+      // board exactly on the seat, so the moment it clicks it pops on top,
+      // and dragging it off drops it back under.
+      style={{ position: 'absolute', left: 0, top: 0, zIndex: seated ? 3 : 0 }}
       onMouseEnter={onMouseEnter}
       onMouseLeave={onMouseLeave}
     >

frontend/src/utils/socketSnap.ts

@@ -93,3 +93,22 @@ export function snapBoardToSocket(
   }
   return best ? { x: tentativeX + best.dx, y: tentativeY + best.dy } : null;
 }
+
+/**
+ * True when the board's CURRENT position IS a socket seat (within half a
+ * pixel). This is the z-order question, not the drag question: a seated
+ * board must paint above its socket component, an unseated one must stay
+ * below components like every other board — the blanket zIndex bump that
+ * preceded this check hid a resistor behind an Arduino in every ordinary
+ * example.
+ */
+export function isBoardSeated(
+  boardId: string,
+  boardKind: string,
+  x: number,
+  y: number,
+  components: ComponentLike[],
+): boolean {
+  const seat = snapBoardToSocket(boardId, boardKind, x, y, components);
+  return !!seat && Math.hypot(seat.x - x, seat.y - y) < 0.5;
+}