MSVC `gte_mask` optimized SSE path is disabled (and the disabled code is buggy)

[protz]

File: krmllib/c/fstar_uint128_msvc.h:362

The && 0 permanently disables the SSE path. The disabled code uses _mm_cmpgt_epi32 which performs signed 32-bit comparison -- incorrect for unsigned 128-bit integers. If someone removes && 0, the bug returns.

// CURRENT:
#if HAS_OPTIMIZED && 0   // permanently disabled -- looks like a debug leftover

Test snippet: (demonstrates the signed comparison bug if the path were enabled)

#include <emmintrin.h>
#include <stdio.h>
int main(void) {
    // 0x80000001 > 0x7FFFFFFF as unsigned, but < as signed
    __m128i a = _mm_set_epi32(0, 0, 0, 0x80000001);
    __m128i b = _mm_set_epi32(0, 0, 0, 0x7FFFFFFF);
    __m128i gt = _mm_cmpgt_epi32(a, b);
    int mask = _mm_movemask_epi8(gt);
    printf("signed cmpgt mask: 0x%x (expect 0xF for unsigned gt, got 0x0)\n", mask);
    return 0;
}

Suggested fix: Add a comment explaining why it's disabled:

#if 0 /* Disabled: _mm_cmpgt_epi32 performs SIGNED comparison, which is
       * incorrect for unsigned 128-bit integers. The scalar fallback below
       * using FStar_UInt64_gte_mask is correct. */

This issue was found by Claude (Opus 4.6)