Non-const `static` variables in headers

[protz]

Files: FStar_UInt128_Verified.h:121,286, fstar_uint128_msvc.h:216,424

static uint32_t FStar_UInt128_u32_64 = 64U;
static uint32_t FStar_UInt128_u32_32 = 32U;

Each translation unit gets its own mutable copy. Should be const to enable constant-folding and prevent accidental modification.

Test snippet:

// This compiles and silently corrupts all subsequent shift operations:
FStar_UInt128_u32_64 = 32;  // Oops -- now all 128-bit shifts are wrong

Suggested fix:

static const uint32_t FStar_UInt128_u32_64 = 64U;
static const uint32_t FStar_UInt128_u32_32 = 32U;

This issue was found by Claude (Opus 4.6)