feat(e10): run on Windows as well as Linux

Serial access moves behind open_port(), with a termios backend on POSIX and a
Win32 backend reached through ctypes. Deliberately no pyserial: these tools
have to run on locked-down, air-gapped shopfloor machines where installing a
package is a project of its own.

Verified on a real controller from both a Linux laptop and a Windows 11 PC.

Three things that made this harder than it should have been, all now handled:

  - CreateFileW returns a HANDLE, which is 64-bit. Without an explicit restype
    ctypes truncates it to c_int. The open then appears to succeed and every
    read and write silently operates on a bogus handle, which looks exactly
    like a dead cable. Every kernel32 call now declares argtypes and restype.

  - Defaulting to a fixed COM number was actively harmful. A wrong-but-real
    port opens without error and answers nothing, so defaulting to COM3 while
    the adapter was on COM4 produced the same silent symptom. Windows now
    auto-picks only when exactly one port looks like a USB virtual COM port,
    and otherwise refuses and lists what it found.

  - COM10 and above need the \\.\ prefix, which is harmless on COM1-9, so it
    is always applied.

Adds two diagnostics, because "no response" has too many possible causes to
guess at:
  ports    every serial port that actually exists. On Windows this reads
           SERIALCOMM from the registry, so a port absent here does not exist
           now, whatever Device Manager showed last week.
  doctor   walks the layers - discovery, open, write, read - and reports which
           one fails, including a warning when the configured port is an
           onboard one while a USB adapter is present.

Port selection is -d, then $E10_PORT, then the safe auto-pick. Friendlier
errors on both platforms for a missing port or a permissions problem.

scripts/simulate-ednc.py

@@ -302,8 +302,10 @@ def main():
                         help="operator input, in prompt order. repeatable.")
     parser.add_argument("--no-clear", action="store_true",
                         help="skip the clear-fields pass")
-    parser.add_argument("--e10", metavar="DEVICE",
-                        help="also drive a real e10 on this serial device")
+    parser.add_argument("--e10", metavar="DEVICE", nargs="?", const="",
+                        help="also drive a real e10. Give a port, or pass the"
+                             " flag bare to use $E10_PORT / the platform"
+                             " default.")
     parser.add_argument("--pattern",
                         help="override the pattern name from the ^ line."
                              " Useful when the e10 file is named differently"
@@ -383,8 +385,9 @@ def drive_e10(seq, args):
     e10mod = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(e10mod)
 
-    print("\n  DRIVING THE REAL CONTROLLER on %s" % args.e10)
-    with e10mod.E10(args.e10, timeout=5.0) as dev:
+    port = args.e10 or e10mod.default_port()
+    print("\n  DRIVING THE REAL CONTROLLER on %s" % port)
+    with e10mod.E10(port, timeout=5.0) as dev:
         for label, raw in seq:
             type_char, data = decode_frame(raw)
             action, payload = translate(type_char, data)