feat(windows): replace php-sdk-binary-tools with MSYS2 + 7za-win

- Add msys2-build-essentials target: downloads the MSYS2 nightly sfx,
  extracts it, disables PGP keyring (CI-safe), runs two-pass pacman
  update, and installs autotools build essentials (make, autoconf,
  automake, libtool, pkgconf, perl).
- Add 7za-win target: downloads 7za.exe to PKG_ROOT_PATH\bin.
- Remove php-sdk-binary-tools target and all PHP_SDK_PATH references;
  replace with SPC_MSYS2_PATH throughout Artifact, ArtifactExtractor,
  FileSystem, DefaultShell and MSVCToolchain.
- Replace {php_sdk_path} path placeholder with {spc_msys2_path}.
- WindowsToolCheck: replace checkSDK/installSDK with checkMsys2,
  installMsys2 and check7zaWin/install7zaWin fix items.
- nasm.yml: extract nasm.exe/ndisasm.exe to {pkg_root_path}/bin.
- env.ini: rename PHP_SDK_PATH to SPC_MSYS2_PATH.
- Docs: update Windows migration guide and package-model placeholder docs.

src/StaticPHP/Runtime/Shell/DefaultShell.php

@@ -184,12 +184,14 @@ class DefaultShell extends Shell
      */
     public function execute7zExtract(string $archive_path, string $target_path): bool
     {
-        $sdk_path = getenv('PHP_SDK_PATH');
-        if ($sdk_path === false) {
-            throw new SPCInternalException('PHP_SDK_PATH environment variable is not set');
+        // 7za.exe is installed by the 7za-win target package into PKG_ROOT_PATH\bin,
+        // which is added to PATH by MSVCToolchain::initEnv().
+        $_7z_path = FileSystem::convertPath(PKG_ROOT_PATH . '\bin\7za.exe');
+        if (!file_exists($_7z_path)) {
+            throw new SPCInternalException('7za.exe not found. Please install the 7za-win target package.');
         }
 
-        $_7z = escapeshellarg(FileSystem::convertPath($sdk_path . '/bin/7za.exe'));
+        $_7z = escapeshellarg(FileSystem::convertPath($_7z_path));
         $archive_arg = escapeshellarg(FileSystem::convertPath($archive_path));
         $target_arg = escapeshellarg(FileSystem::convertPath($target_path));