Add Windows thread affinity support

[GagaLP]

Thread pinning was previously a no-op on Windows: thread_pinner only emitted a warning ("Thread pinning is currently not supported on Windows.") and did not apply any affinity settings, regardless of configuration.

This PR adds a real implementation by introducing a small pthread/cpu_set_t compatibility layer (platform_specific/affinity_win32.h/.cc). The layer maps the POSIX affinity APIs already used by Celerity (sched_getaffinity/sched_setaffinity, pthread_self, pthread_get/setaffinity_np, CPU_SET/CPU_ISSET/CPU_COUNT, etc.) to their Windows equivalents (GetActiveProcessorGroupCount, GROUP_AFFINITY, SetThreadGroupAffinity).

With this compatibility layer in place, affinity_win.cc no longer requires separate pinning logic and now closely mirrors the Linux implementation. It initializes and tears down the pinning plan in the same way and pins threads to sequential cores using the same approach.

One limitation to note: Windows organizes logical processors into groups of up to 64 processors, and a single GROUP_AFFINITY mask cannot span multiple groups. If a requested core set crosses a group boundary, the layer currently emits a warning and skips pinning instead of attempting cross-group affinity. This should not affect the common case of pinning a small number of sequential cores, but remains a limitation on systems with more than 64 logical processors.

Added corresponding tests in affinity_tests.cc.

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Check-perf-impact results: (ae6918621b46271c2f10d6eb978fe95d)

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