port clide runtime to Windows (ConPTY, AF_UNIX, PATHEXT)

Bring the runtime up on Windows without disturbing the POSIX paths.

PTY: introduce a platform-neutral PtySession contract with a factory
that picks NativePty (posix_openpt/posix_spawn) or the new WindowsPty
(ConPTY via CreatePseudoConsole). The pane registry programs against
the interface; NativePty now implements it.

IPC: the per-workspace AF_UNIX socket lives under %LOCALAPPDATA% and
is hashed from a canonical workspace key (backslash + ASCII-folded
case) so the Dart server and the C client agree despite NTFS case-
insensitivity. The C client grows a Win32 shim (winsock afunix);
chmod is a no-op on Windows where the per-user ACL is the gate.

Toolchain: PATH probing splits on ';' and tries PATHEXT extensions;
the shell defaults to PowerShell (pwsh, then powershell); tmux is
treated as optional since it has no Windows build; dugite falls back
to PATH git for now.

Build: add `make build-windows`, a clide-cli MSVC build wrapped by
ci/build_cli_windows.sh, and a ConPTY smoke-test suite that self-
skips off-platform.

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

lib/kernel/src/toolchain.dart

@@ -10,6 +10,7 @@
 library;
 
 import 'dart:async';
+import 'dart:io' show Platform;
 
 import 'package:flutter/foundation.dart';
 
@@ -32,7 +33,7 @@ class Toolchain extends ChangeNotifier implements ToolchainView {
   @override
   String get tmux => _tmux ?? 'tmux';
   @override
-  String get shell => _shell ?? '/bin/bash';
+  String get shell => _shell ?? (Platform.isWindows ? 'powershell.exe' : '/bin/bash');
 
   /// Extra environment variables for git (e.g. GIT_EXEC_PATH for dugite).
   @override
@@ -44,7 +45,13 @@ class Toolchain extends ChangeNotifier implements ToolchainView {
   bool get allOk => _resolved && missing.isEmpty;
 
   @override
-  List<String> get missing => [if (_git == null) 'git', if (_pql == null) 'pql', if (_tmux == null) 'tmux'];
+  List<String> get missing => [
+    if (_git == null) 'git',
+    if (_pql == null) 'pql',
+    // tmux has no Windows build; its absence there is the documented
+    // no-tmux mode, not a missing tool.
+    if (_tmux == null && !Platform.isWindows) 'tmux',
+  ];
 
   /// Returns a Future that completes when resolution finishes.
   Future<void> waitForResolution() {