esp32s3: replace inline ISR with full interrupt vector handler

Replace the minimal inline ISR (which only disabled INTENABLE) with a
full level-1 interrupt handler that saves/restores the interrupted
context and dispatches to Go's handleInterrupt.

The handler uses callx4 (not callx0) to call into Go code because:
- callx0 does not set PS.CALLINC, so the Go function's entry
  instruction uses stale CALLINC from the interrupted code, causing
  wrong window rotation and a garbage stack pointer.
- callx4 explicitly sets CALLINC=1, and our frame pointer (a1) is
  outside the callee's register window so it is preserved.

Also updates the USB Serial/JTAG ISR to disable INT_ENA (peripheral
level) instead of relying on INTENABLE, and adds signalInterrupt to
the dispatcher so sleepTicks can be woken by any interrupt.

src/machine/machine_esp32xx_usb.go

@@ -112,10 +112,11 @@ func (usbdev *USB_DEVICE) ensureConfigured() {
 }
 
 // handleInterrupt is called from the CPU interrupt vector when the USB
-// peripheral raises an interrupt.  For now, just clear the interrupt flag.
-// The actual data drain happens in Buffered() via polling — once the ISR
-// mechanism is proven, we can move the drain here.
+// peripheral raises an interrupt.  Disable INT_ENA to prevent the
+// level-triggered interrupt from re-asserting immediately (data may
+// still be in the FIFO).  Buffered() re-enables after draining.
 func (usbdev *USB_DEVICE) handleInterrupt() {
+	usbdev.Bus.SetINT_ENA_SERIAL_OUT_RECV_PKT_INT_ENA(0)
 	usbdev.Bus.SetINT_CLR_SERIAL_OUT_RECV_PKT_INT_CLR(1)
 }
 
@@ -162,7 +163,7 @@ func (usbdev *USB_DEVICE) Write(data []byte) (n int, err error) {
 
 // Buffered returns the number of bytes waiting in the receive ring buffer.
 // It drains any data sitting in the hardware FIFO and re-enables the
-// USB interrupt (which the ISR disables via INTENABLE to prevent a
+// peripheral-level USB interrupt (which the ISR disables to prevent a
 // level-triggered interrupt storm).
 func (usbdev *USB_DEVICE) Buffered() int {
 	usbdev.ensureConfigured()
@@ -171,11 +172,9 @@ func (usbdev *USB_DEVICE) Buffered() int {
 		b := byte(usbdev.Bus.EP1.Get())
 		usbdev.Buffer.Put(b)
 	}
-	// Clear pending flags and re-enable the RX interrupt.
+	// Clear pending flags and re-enable the RX interrupt at the peripheral level.
 	usbdev.Bus.INT_CLR.Set(0xFFFFFFFF)
 	usbdev.Bus.SetINT_ENA_SERIAL_OUT_RECV_PKT_INT_ENA(1)
-	// Re-enable CPU interrupt 8 in INTENABLE (the ISR clears all bits).
-	interrupt.New(cpuInterruptFromUSB, usbHandleInterrupt).Enable()
 	return int(usbdev.Buffer.Used())
 }
 

src/runtime/interrupt/interrupt_esp32s3.go

@@ -115,6 +115,9 @@ func handleInterrupt() {
 		}
 	}
 
+	// Signal to sleepTicks that an interrupt has occurred.
+	signalInterrupt()
+
 	inInterrupt = false
 }
 
@@ -180,6 +183,9 @@ func callHandler(n int) {
 //go:linkname callHandlers runtime/interrupt.callHandlers
 func callHandlers(num int)
 
+//go:linkname signalInterrupt runtime.signalInterrupt
+func signalInterrupt()
+
 var errInterruptRange = constError("interrupt for ESP32-S3 must be in range 6 through 30")
 
 type constError string