✨ feat: UART monitor, step timeouts, and non-blocking subprocess output

Based on real hardware timing data from XC7Z100 testing:

UART Monitoring:
- New UartMonitor class in serial_monitor.py: background thread reads
  serial output continuously, parses for IP/version/boot messages
- GUI starts monitor when workflow runs, stops on completion/close
- Serial lines flow into log display in real time

Step Timing:
- New step_timeouts config (experience × 2):
  check_env:30s flash_erase:120s flash_program:600s
  bitstream:60s elf_download:60s tftp_reboot:120s
- GUI shows estimated timeout before each step
- Shows actual elapsed time after each step completes
- All subprocess calls now use config-driven timeouts

Non-blocking Output:
- New subprocess_utils.py: run_streaming() uses Popen + threaded
  stdout/stderr readers, parses WARNING/ERROR/progress% in real time
- flash_programmer and bitstream_programmer use it
- Callbacks receive categorized events (out/err/warn/error/progress/done)

src/gui/main_window.py

@@ -24,7 +24,7 @@ from ip_verifier import ping_ip
 from tftp_manager import tftp_upload_verify, TftpResult
 from reboot_manager import reboot_zynq, RebootResult
 from boot_verifier import verify_boot, verify_zynq_status, BootVerificationResult
-from serial_monitor import detect_serial_ports, parse_boot_output, check_uart_available
+from serial_monitor import detect_serial_ports, parse_boot_output, check_uart_available, UartMonitor
 from gui.styles import (
     WINDOW_WIDTH,
     WINDOW_HEIGHT,
@@ -79,6 +79,7 @@ class MainWindow(ctk.CTk):
         self._uart_message: str = ""
         self._zynq_jtag_present: bool = False
         self._zynq_jtag_info = None
+        self._uart_monitor: UartMonitor | None = None
 
         # StringVar references for config sync
         self._ip_string_var: ctk.StringVar | None = None
@@ -961,6 +962,15 @@ class MainWindow(ctk.CTk):
             self._run_step_3_load_bootloader,
             self._run_step_4_load_main_program,
         ]
+        step_timeout_keys = [
+            ("check_env", 30),
+            ("flash_program", 600),
+            ("bitstream", 60),
+            ("tftp_reboot", 120),
+        ]
+
+        # Start UART monitor if available
+        self._start_uart_monitor()
 
         # Determine end index based on single_step flag
         end_index = start_index + 1 if single_step else len(step_funcs)
@@ -970,12 +980,21 @@ class MainWindow(ctk.CTk):
         for i in range(start_index, end_index):
             step_idx = i - start_index
             self._set_step_status(i, "running")
+
+            # Log step timeout estimate
+            timeout_key, default_timeout = step_timeout_keys[i] if i < len(step_timeout_keys) else ("", 30)
+            step_timeout = self._config.step_timeouts.get(timeout_key, default_timeout) if self._config else default_timeout
+            self._log_message(f"  ⏱ Step {i+1} timeout: {step_timeout}s")
+
             self._progress.set_value(step_idx / total)
+            t_step_start = time.time()
 
             try:
                 success = step_funcs[i]()
                 results.append(success)
                 self._set_step_status(i, "complete" if success else "error")
+                elapsed = time.time() - t_step_start
+                self._log_message(f"  ⏱ Step {i+1} completed in {elapsed:.0f}s")
             except Exception as e:
                 results.append(False)
                 self._set_step_status(i, "error")
@@ -1020,6 +1039,35 @@ class MainWindow(ctk.CTk):
         # Save config after running
         self._save_config()
 
+        # Stop UART monitor
+        self._stop_uart_monitor()
+
+    # ── UART Monitor ──────────────────────────────────────────
+
+    def _start_uart_monitor(self) -> None:
+        """Start background serial monitoring if UART is available."""
+        if not self._uart_available or not self._config:
+            return
+        port = self._config.serial_port
+        if not port:
+            return
+
+        self._uart_monitor = UartMonitor(port, self._config.serial_baudrate)
+        self._uart_monitor.on_line = lambda line: self._log_message(f"  [UART] {line}")
+        self._uart_monitor.on_boot_info = lambda info: (
+            self._log_message(f"  [UART] Boot detected: IP={info.ip_address}, Ver={info.version}")
+            if info.ip_address or info.version else None
+        )
+        self._uart_monitor.on_error = lambda e: self._log_message(f"  [UART] Error: {e}")
+        if self._uart_monitor.start():
+            self._log_message("  [UART] Serial monitor started")
+
+    def _stop_uart_monitor(self) -> None:
+        """Stop background serial monitoring."""
+        if self._uart_monitor:
+            self._uart_monitor.stop()
+            self._uart_monitor = None
+
     # ── Serial ─────────────────────────────────────────────────
 
     def _refresh_ports(self) -> None:
@@ -1040,6 +1088,7 @@ class MainWindow(ctk.CTk):
 
     def _on_close(self) -> None:
         """Handle window close event: save config and cleanup."""
+        self._stop_uart_monitor()
         if self._config:
             self._sync_config_from_ui()  # Sync UI values first
             # Save to user config file (config.yaml)