Windows support (x64)

[lisachenko]

Goal

Support running z-engine on Windows x64. The runtime resolves FFI definitions by platform key, so the work is generating and validating windows-x64 artifacts and handling the Windows-specific symbol loading.

What's needed

• Header generation on Windows. emit.php relies on clang, cpp/clang -E, php-config and a C compiler. On Windows this needs the PHP SDK toolchain (or clang-cl) and the PHP dev pack headers. generate.php's Docker orchestration is Linux-only; a Windows path (native or a Windows CI runner) is required.
• Symbol loading. Unlike Linux/macOS (which resolve engine symbols from the running process), Windows needs FFI_LIB pointing at the PHP DLL (historically php7.dll; for PHP 8 it's php8ts.dll/php8.dll depending on the build). The generated header currently only emits FFI_SCOPE; the Windows generator will need to emit the correct FFI_LIB line, and Core will need to select it.
• Calling convention. ZEND_FASTCALL maps to __vectorcall on Windows — verify the generated declarations handle this (the current generator strips it via cpp, which may or may not be correct for the ABI).
• ZTS. Official Windows PHP builds are commonly thread-safe; this likely needs the ZTS runtime work (separate issue) as a prerequisite.

Acceptance

• Generated include/8.4/windows-x64-*/* validated against a real Windows PHP 8.4 build
• Symbol resolution via the correct PHP DLL
• A Windows CI job runs at least the non-destructive suite

Part of the platform-support roadmap referenced in the README support matrix (see #57).