Port serial layer to pyserial; add Windows COM detection and PyInstaller spec

- serial_port.py: replace termios/fcntl/select with pyserial wrapper,
  same SerialPort interface preserved for all other modules
- workflow_serial.py: detect_serial_device() uses serial.tools.list_ports
  to enumerate COM ports; removes _fix_permissions() (not needed on Windows);
  multi-device table now shows port description
- es24n_conf.py: enable VT/ANSI colour mode on Windows 10+ via ctypes;
  update docstring for Windows edition
- requirements.txt: pyserial >= 3.5, pyinstaller >= 6.0
- es24n_conf.spec: PyInstaller single-file .exe spec

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

es24n_conf.spec

@@ -0,0 +1,43 @@
+# PyInstaller spec for es24n_conf Windows .exe
+# Build with: pyinstaller es24n_conf.spec
+
+from PyInstaller.building.build_main import Analysis, PYZ, EXE
+
+a = Analysis(
+    ["es24n_conf.py"],
+    pathex=[],
+    binaries=[],
+    datas=[
+        ("modules", "modules"),
+    ],
+    hiddenimports=[
+        "serial",
+        "serial.tools",
+        "serial.tools.list_ports",
+    ],
+    hookspath=[],
+    hooksconfig={},
+    runtime_hooks=[],
+    excludes=[],
+    noarchive=False,
+)
+
+pyz = PYZ(a.pure)
+
+exe = EXE(
+    pyz,
+    a.scripts,
+    a.binaries,
+    a.datas,
+    [],
+    name="es24n_conf",
+    debug=False,
+    bootloader_ignore_signals=False,
+    strip=False,
+    upx=True,
+    console=True,       # console app — must stay True for terminal UI
+    disable_windowed_traceback=False,
+    target_arch=None,
+    codesign_identity=None,
+    entitlements_file=None,
+)