✨ feat(uart): add UART availability check, inter-step delays, and post-download Zynq status verification

- Add check_uart_available() to serial_monitor.py — detects port availability
- Add verify_zynq_status() to boot_verifier.py — lightweight post-download check
- Add uart_delay / inter_step_delay config fields (defaults: 3s / 2s)
- Step 1: Check UART availability, show non-blocking 'UART 不可用' notification
- Step 4: Add post-download Zynq status check (ping + serial) after TFTP
- _run_steps_worker: Add configurable inter-step delays between all steps
- Update step labels (4a→4b→4c→4d) for clarity

src/serial_monitor.py

@@ -108,6 +108,56 @@ def parse_boot_output(lines: list[str]) -> BootInfo:
     return info
 
 
+def check_uart_available(
+    port: str,
+    baudrate: int = 115200,
+    timeout: float = 3.0,
+) -> tuple[bool, str]:
+    """Check whether the UART serial port is available and readable.
+
+    Attempts to open the port, clear buffers, and read a few lines.
+    If the port opens but returns no data within the timeout, it is
+    considered available (the device may simply not be outputting yet).
+    If the port cannot be opened at all, it is unavailable.
+
+    Args:
+        port: Serial port device path (e.g., '/dev/ttyUSB0').
+        baudrate: Baud rate for serial communication.
+        timeout: Seconds to wait for data after opening.
+
+    Returns:
+        Tuple of (is_available, reason_string).
+        - (True,  "Port open") — port is usable
+        - (True,  "Port open, no data yet") — port open, no data currently
+        - (False, "No serial ports detected") — system has no serial ports
+        - (False, "Port not in detected ports") — configured port missing
+        - (False, "Failed to open: <error>") — port open error
+    """
+    # 1. Quick port list check
+    ports = detect_serial_ports()
+    if not ports:
+        return False, "No serial ports detected"
+
+    # 2. Verify the configured port is among detected ones
+    if port and port not in {p.device for p in ports}:
+        return False, f"Port {port} not in detected ports"
+
+    # 3. Try to open and read
+    try:
+        with serial.Serial(port, baudrate, timeout=timeout) as ser:
+            ser.reset_input_buffer()
+            lines: list[str] = []
+            while ser.in_waiting:
+                line = ser.readline().decode("utf-8", errors="replace").strip()
+                if line:
+                    lines.append(line)
+        if lines:
+            return True, "Port open"
+        return True, "Port open, no data yet"
+    except serial.SerialException as e:
+        return False, f"Failed to open: {e}"
+
+
 def read_serial_stream(
     port: str,
     baudrate: int = 115200,