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.

config/pkg/target/7za-win.yml

@@ -0,0 +1,5 @@
+7za-win:
+  type: target
+  artifact:
+    binary:
+      windows-x86_64: { type: url, url: 'https://dl.static-php.dev/v3/tools/7zip/7za.exe', extract: '{pkg_root_path}/bin/7za.exe' }