stillwater-sc / universal / 24939331552

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feat(internal): blockbinary mul/div/mod constexpr foundation (#760)

* feat(internal): blockbinary mul/div/mod constexpr foundation

PR #716 closed out the easy half of blockbinary constexpr work
(add/sub/shift/bitwise). The mul/div/mod path was deferred because the
multi-limb uint64_t-block fast-mul uses platform intrinsics (mul128,
addcarry) that are not constexpr on MSVC.

This PR completes the constexpr surface using the same pattern just
landed for integer<> in PR #757: std::is_constant_evaluated() dispatch
with a __uint128_t-based portable fallback (gcc/clang only) for the
uint64-limb mul, while keeping the intrinsic path at runtime for
performance.

blockbinary.hpp changes:
  - operator*= (FAST_MUL path): constexpr decoration; is_constant_evaluated
    dispatch wraps both the signed and unsigned bitsInBlock==64 branches;
    portable __uint128_t carry-propagation fallback gated on
    __SIZEOF_INT128__
  - operator*= (slow shift-and-add #else path): constexpr decoration
  - operator/=, operator%=: constexpr (single-block fast path was already
    constexpr-clean; multi-limb path goes through longdivision)
  - longdivision() and its forward decl: constexpr
  - msb(), to_sll(), roundingMode(), any(): constexpr
  - operator-() unary, operator~() (both overloads): constexpr
  - cross-template copy ctor and assign()/assignWithoutSignExtend():
    constexpr (required by urmul2 which calls blockbinary<N+1>(a))
  - free urmul, urmul2, uradd, ursub: constexpr
  - free operator*, operator/, operator%: constexpr

internal/blockbinary/api/constexpr.cpp:
  - Added "mul/div/mod (issue #759)" section covering:
    - single-block mul (uint8 limb)
    - multi-limb mul (uint8 and uint32 limbs)
    - uint64-limb __int128 carry path including a 2^32 * 2^32 = 2^64
      cross-limb carry test
    - single-block and multi-limb div/mod
    - free operator*, /, % constexpr wrappers
    - divide-by-zero defined behavior (returns zero, no UB)
    - uradd/ursub/urmul2 con... (continued)

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