✨ 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/bitstream_programmer.py

@@ -117,21 +117,17 @@ def _run_command(
     Raises:
         subprocess.TimeoutExpired: If command exceeds timeout.
     """
+    from subprocess_utils import run_streaming
+
     if callback:
         callback("running", f"Executing: {' '.join(cmd)}")
 
-    result = subprocess.run(
-        cmd,
-        capture_output=True,
-        text=True,
-        timeout=timeout,
-    )
-    if callback:
-        if result.returncode == 0:
-            callback("complete", f"{cmd[0]} succeeded")
-        else:
-            callback("error", f"{cmd[0]} failed (exit {result.returncode})")
-    return result
+    try:
+        return run_streaming(cmd, timeout=timeout, callback=callback)
+    except subprocess.TimeoutExpired:
+        if callback:
+            callback("error", f"{cmd[0]} timed out after {timeout}s")
+        raise
 
 
 # ── Bitstream Download ──────────────────────────────────────────────
@@ -172,7 +168,8 @@ def program_bitstream(
     # Try xsct first (preferred)
     xsct = get_xsct_path(config.vitis_path)
     if xsct:
-        return _program_with_xsct(xsct, bit_path, callback, ps7_tcl)
+        timeout = config.step_timeouts.get("bitstream", 60)
+        return _program_with_xsct(xsct, bit_path, callback, ps7_tcl, timeout)
 
     # Fallback to Vivado
     vivado = _get_vivado_path(config.vitis_path)
@@ -191,6 +188,7 @@ def _program_with_xsct(
     bit_path: Path,
     callback: ProgressCallback = None,
     ps7_init_tcl: str = "",
+    timeout: int = 60,
 ) -> BitstreamResult:
     """Program bitstream using xsct's fpga command.
 
@@ -203,6 +201,7 @@ def _program_with_xsct(
         bit_path: Path to .bit file.
         callback: Optional progress callback.
         ps7_init_tcl: Path to ps7_init.tcl for PS initialization.
+        timeout: Subprocess timeout in seconds.
 
     Returns:
         BitstreamResult with status.
@@ -232,7 +231,7 @@ def _program_with_xsct(
             [xsct_path, tcl_file],
             capture_output=True,
             text=True,
-            timeout=120,
+            timeout=timeout,
         )
         os.unlink(tcl_file)
         success = result.returncode == 0
@@ -351,7 +350,8 @@ def run_elf(
 
     # Auto-detect ps7_init.tcl from config or BIT file directory
     ps7_tcl = _find_ps7_init_tcl(config)
-    return _run_elf_with_xsct(xsct, elf_path, callback, ps7_tcl)
+    timeout = config.step_timeouts.get("elf_download", 60)
+    return _run_elf_with_xsct(xsct, elf_path, callback, ps7_tcl, timeout)
 
 
 def _run_elf_with_xsct(
@@ -359,6 +359,7 @@ def _run_elf_with_xsct(
     elf_path: Path,
     callback: ProgressCallback = None,
     ps7_init_tcl: str = "",
+    timeout: int = 60,
 ) -> BitstreamResult:
     """Download and run ELF using xsct's dow command.
 
@@ -399,7 +400,7 @@ def _run_elf_with_xsct(
             [xsct_path, tcl_file],
             capture_output=True,
             text=True,
-            timeout=120,
+            timeout=timeout,
         )
         os.unlink(tcl_file)