fix(multi-board): initSimulator wiped Interconnect's UART wrapper

Cross-board UART forwarding silently broke for any project loaded
with > 1 board. User report: Arduino Uno → Arduino Nano serial echo
test where the Uno transmits fine but the Nano's Serial.available()
is never true.

Root cause traced live with chrome-devtools-mcp + temporary debug
logs in AVRSimulator.onSerialData setter and Interconnect:

  1. loadProjectState → addBoard(uno) → createSimulator → sim.onSerialData = appendSerial
  2. addBoard(nano) → same
  3. setWires → Interconnect.updateWires → ensureSerialHook(uno)
     wraps sim.onSerialData with a fan-out callback that ALSO pushes
     to the Nano's RX queue. __icSerialHookInstalled flag set.
  4. SimulatorCanvas mounts → useEffect calls store.initSimulator()
  5. initSimulator unconditionally did:
        simulatorMap.delete(boardId);
        const sim = createSimulator(...);   // ← brand-new sim
        simulatorMap.set(boardId, sim);     // ← Interconnect's wrapper is gone
     The new sim's onSerialData is just appendSerial. The old sim
     (where the wrapper lived) has been orphaned; Interconnect never
     re-installs because its flag was on the discarded sim.
  6. Run all boards → Uno.usart.onByteTransmit → this.onSerialData →
     appendSerial (Uno's monitor shows TX) but no fan-out call →
     Nano never receives anything.

initSimulator is a legacy single-board helper from the days when the
store only knew about one MCU. Multi-board flows already create
their sims in addBoard. Bail out early if a sim for the active
boardId already exists, so the legacy helper becomes a no-op when
the multi-board path has already done the work.

Verified the 3 related test suites still pass (AVRSimulator,
dual-arduino-software-serial, interconnect-routing).

frontend/src/simulation/AVRSimulator.ts

@@ -301,13 +301,7 @@ export class AVRSimulator {
   private scheduledPinChanges: Array<{ cycle: number; pin: number; state: boolean }> = [];
 
   /** Serial output buffer — subscribers receive each byte or line */
-  private _onSerialData: ((char: string) => void) | null = null;
+  public onSerialData: ((char: string) => void) | null = null;
-  public get onSerialData(): ((char: string) => void) | null { return this._onSerialData; }
-  public set onSerialData(v: ((char: string) => void) | null) {
-    const who = new Error().stack?.split('\n').slice(1, 6).join(' | ');
-    console.log('[AVR-DBG] onSerialData SET (fn?', typeof v === 'function', ') from', who);
-    this._onSerialData = v;
-  }
   /** Fires whenever the sketch changes Serial baud rate (Serial.begin) */
   public onBaudRateChange: ((baudRate: number) => void) | null = null;
   /**

frontend/src/simulation/Interconnect.ts

@@ -174,13 +174,12 @@ function pushPinState(boardId: string, pin: number, state: boolean): void {
 
 /** Push a UART byte into the receiving board's UART RX. */
 function pushSerialByte(boardId: string, ch: string, uart: number): void {
-  if (!runtime) { console.log('[IC-DBG] pushSerialByte: no runtime'); return; }
+  if (!runtime) return;
   const entry = boards.get(boardId);
-  if (!entry) { console.log('[IC-DBG] pushSerialByte: no entry for', boardId); return; }
+  if (!entry) return;
 
   if (isBrowserSim(entry.kind)) {
     const sim = runtime.getBoardSimulator(boardId);
-    console.log('[IC-DBG] pushSerialByte to', boardId, 'ch=', JSON.stringify(ch), 'sim=', !!sim, 'feedUart=', !!sim?.feedUart, 'serialWrite=', !!sim?.serialWrite);
     // RP2040Simulator doesn't yet expose feedUart per-UART — fall back
     // to serialWrite (which feeds UART0) for uart === 0.
     if (sim?.feedUart) {
@@ -287,7 +286,6 @@ function ensureSerialHook(entry: BoardEntry): void {
     if ((sim as any).__icSerialHookInstalled) return;
     (sim as any).__icSerialHookInstalled = true;
     entry.origSerialCallback = sim.onSerialData ?? null;
-    console.log('[IC-DBG] ensureSerialHook installed on', boardId, 'origCb=', !!entry.origSerialCallback);
     sim.onSerialData = (ch: string, uart?: number) => {
       const liveEntry = boards.get(boardId);
       liveEntry?.origSerialCallback?.(ch, uart);
@@ -295,7 +293,6 @@ function ensureSerialHook(entry: BoardEntry): void {
       // same callback. Default to UART0 for routing.
       const u = uart ?? 0;
       const subs = liveEntry?.serialFanout.get(u);
-      console.log('[IC-DBG] wrapper invoked for', boardId, 'ch=', JSON.stringify(ch), 'subs=', subs ? subs.size : 'none');
       if (subs) for (const cb of subs) cb(ch);
     };
     return;
@@ -388,7 +385,6 @@ function buildRouteForWire(wire: Wire): RouteHandle | null {
     const aRoleIsTx = classifyPin(aEntry.kind, wire.start.pinName).kind === 'uart-tx';
     const aUart = uartInfo.uartA;
     const bUart = uartInfo.uartB;
-    console.log('[IC-DBG] UART route built:', aEntry.id, '(tx?', aRoleIsTx, ')', '<->', bEntry.id, 'uartA=', aUart, 'uartB=', bUart);
     if (aRoleIsTx) {
       // A.TX → B.RX
       teardowns.push(installSerialFanout(aEntry.id, aUart, bEntry.id, bUart));

frontend/src/store/useSimulatorStore.ts

@@ -1787,8 +1787,18 @@ export const useSimulatorStore = create<SimulatorState>((set, get) => {
       const boardId = activeBoardId ?? INITIAL_BOARD_ID;
       const pm = getBoardPinManager(boardId) ?? legacyPinManager;
 
-      getBoardSimulator(boardId)?.stop();
+      // Multi-board flows (addBoard, loadProjectState) already create
-      simulatorMap.delete(boardId);
+      // sims + register them in simulatorMap, AND Interconnect wraps
+      // sim.onSerialData for cross-board UART forwarding. SimulatorCanvas
+      // runs initSimulator() once on mount as a legacy single-board
+      // "make sure a sim exists for the active board" helper. If we let
+      // it through here when a sim ALREADY exists we wipe simulatorMap,
+      // recreate the sim, and silently drop the Interconnect wrapper —
+      // every cross-board wire stops forwarding bytes (Nano never sees
+      // anything the Uno sends). Skip out early in that case.
+      const existingSim = getBoardSimulator(boardId);
+      if (existingSim) return;
+
       getEsp32Bridge(boardId)?.disconnect();
       esp32BridgeMap.delete(boardId);