fix: replace __uint128_t with portable 64-bit decomposition in c_shard_info.pyx (MSVC build failure)

[mykaul]

Bug

cassandra/c_shard_info.pyx computes the shard id for a token by taking the
high 64 bits of a 64x32-bit product:

cdef int shardId = (<__uint128_t>biased_token * self.shards_count) >> 64;

__uint128_t is a GCC/Clang compiler-builtin extension type — it is not part
of the C or C++ standard, and it is not a type Cython itself understands
natively (the cdef extern from *: ctypedef unsigned int __uint128_t block
just tells Cython to trust that the C compiler has it). MSVC has no 128-bit
integer type at all, so on Windows the generated c_shard_info.c fails to
compile with error C2065: '__uint128_t': undeclared identifier, followed by
a cascade of C2146/C2059 syntax errors. This breaks Windows wheel builds for
any change that touches (or merely triggers a rebuild of) this extension.

Fix

Replace the 128-bit multiply with a portable multiply-high decomposition that
uses only 64-bit arithmetic (uint64_t), splitting the multiplication into
32-bit halves — the same technique already used by the pure-Python fallback
in cassandra/shard_info.py (_ShardingInfo.shard_id_from_token). This
compiles identically on GCC, Clang, and MSVC.

Verification

• Built the extension locally with Cython + gcc and confirmed it compiles
  cleanly.
• Directly compared the compiled extension's shard_id_from_token output
  against the pure-Python fallback (cassandra.shard_info._ShardingInfo)
  across 2,000,000 randomized (shards_count, sharding_ignore_msb, token)
  triples, covering the full int64 token range — 0 mismatches.
• Ran the full tests/unit/ suite (770 passed, 38 skipped, 0 failed) plus
  the sharding-specific tests (tests/unit/test_shard_aware.py,
  tests/unit/test_connection.py::TestShardawarePortGenerator,
  tests/unit/test_host_connection_pool.py) — all passing, with the real
  compiled extension in place.

Context

This was originally found while investigating a Windows wheel-build CI
failure on an unrelated PR (#806, a ResponseFuture.__init__ cleanup). The
__uint128_t bug is pre-existing, independent of that PR's changes, and
would affect the Windows build of any PR that happens to trigger a rebuild
of this extension — so it's being fixed here as its own standalone, focused
PR rather than folded into #806.

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[Copilot]

Pull request overview

Replaces a compiler-specific 128-bit multiplication with a portable 64-bit decomposition for shard calculation.

Changes:

• Removes the unsupported __uint128_t declaration.
• Computes the product’s high bits using portable 64-bit arithmetic.

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