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/env.ini

@@ -68,8 +68,8 @@ SPC_PRESERVE_LOGS="no"
 [windows]
 ; build target: win7-static
 SPC_TARGET=native-windows
-; php-sdk-binary-tools path
-PHP_SDK_PATH="${WORKING_DIR}\php-sdk-binary-tools"
+; MSYS2 root directory (msys64 subfolder), used by the Windows toolchain
+SPC_MSYS2_PATH="${PKG_ROOT_PATH}\msys2-build-essentials\msys64"
 ; upx executable path
 UPX_EXEC="${PKG_ROOT_PATH}\bin\upx.exe"
 ; phpmicro patches, for more info, see: https://github.com/easysoft/phpmicro/tree/master/patches