Fix UB in PopcountLogic on MSVC

[jamierpond]

PopcountLogic<LogarithmOfGroupSize, T>::CombiningMask computed
T(1 << HalvedGroupSize) - 1, which performs the shift at int
precision because 1 is an int. For LogarithmOfGroupSize = 6,
HalvedGroupSize is 32 — so the expression is 1 << 32, which is UB
per [expr.shift] (shift count not less than width of the promoted type).

GCC happens to accept this in a constant-expression context; Clang and
MSVC /std:c++20 /permissive- correctly reject it ("non-type template
argument is not a constant expression", "shift count 32 >= width of
type 'int' (32 bits)"). The result is that any translation unit that
includes <zoo/swar/SWAR.h> under MSVC with conformance mode fails to
compile, because SWAR.h's unconditional static_asserts on line 589-596
force instantiation of PopcountLogic<6, uint64_t> via logarithmFloor.

Fix: do the shift at T's precision — T(T(1) << HalvedGroupSize) - 1.

Add a regression test that directly instantiates PopcountLogic<6, u64>.
popcount() would have caught this, but on non-MSVC platforms
it routes through PopcountIntrinsic (which uses __builtin_popcountll)
and never touches PopcountLogic<6>, which is why the existing popcount
tests at <1>, <2>, <4>, <5> all passed.