✨ 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/config_manager.py

@@ -31,6 +31,14 @@ DEFAULT_CONFIG: dict[str, Any] = {
     "ping_count": 3,
     "uart_delay": 3,
     "inter_step_delay": 2,
+    "step_timeouts": {
+        "check_env": 30,
+        "flash_erase": 120,
+        "flash_program": 600,
+        "bitstream": 60,
+        "elf_download": 60,
+        "tftp_reboot": 120,
+    },
 }
 
 
@@ -58,6 +66,7 @@ class Config:
     ping_count: int = 3
     uart_delay: int = 3
     inter_step_delay: int = 2
+    step_timeouts: dict[str, int] = field(default_factory=lambda: {})
 
     # Internal: path to the config file on disk
     _config_path: Path = field(default=None, init=False, repr=False)
@@ -163,6 +172,7 @@ class Config:
             "ping_count": self.ping_count,
             "uart_delay": self.uart_delay,
             "inter_step_delay": self.inter_step_delay,
+            "step_timeouts": self.step_timeouts,
         }
         return data