Replace MSVC lazy initialization with DllMain entry point

[Copilot]

On MSVC, all dispatch function pointers were lazily initialized via NULL checks on every call. GCC/Clang use __attribute__((constructor)) for load-time init, but MSVC had no equivalent—just runtime overhead on every invocation.

Changes

• Unified DECLARE_INTERNAL_* and DECLARE_ALL_KEYVALUE_METHODS macros — removed the MSVC-specific variants with NULL checks and forward declarations, keeping only the direct-call versions (~130 lines of duplicated macro code deleted)
• Added XSS_POST_RESOLVE macro — on MSVC, each DISPATCH/DISPATCH_KV_FUNC registers its resolve function into a static array during C++ static init; no-op on other compilers
• Added DllMain on DLL_PROCESS_ATTACH — iterates registered resolve functions and calls them all at load time

// MSVC: resolve functions register themselves during static init
#define XSS_POST_RESOLVE(resolve_func_name) \
    static int CAT(xss_reg_, resolve_func_name) \
            = (xss_init_funcs[xss_init_count < XSS_MAX_INIT_FUNCS \
                                      ? xss_init_count++ \
                                      : xss_init_count] \
                       = &resolve_func_name, 0);

// DllMain calls them all on load
extern "C" BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved)
{
    if (fdwReason == DLL_PROCESS_ATTACH) {
        for (int i = 0; i < xss_init_count; i++)
            xss_init_funcs[i]();
    }
    return TRUE;
}

CRT static initialization order within a single TU guarantees all registrations complete before DllMain is called by _DllMainCRTStartup.

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