Add Windows (clang-cl) support for continuous fuzzing

[posidron]

Summary

Enable continuous fuzzing (--fuzz=) on Windows with clang-cl.

Problem

FuzzTest's continuous fuzzing mode fails on Windows with:

Despite clang-cl correctly applying -fsanitize-coverage=inline-8bit-counters and the __sanitizer_cov_8bit_counters_init callback firing, the coverage data is never used because FUZZTEST_COVERAGE_IS_AVAILABLE excludes _WIN32.

CorpusCoverage::Update() compiles as a stub returning false on Windows, so the fuzzer never considers any input as providing new coverage.

Changes

1. Add defined(_WIN32) to the FUZZTEST_COVERAGE_IS_AVAILABLE guard - clang-cl supports the same sanitizer coverage instrumentation as clang on Linux/macOS.

2. Use _aligned_malloc/_aligned_free on Windows — the MSVC CRT does not provide std::aligned_alloc. Memory allocated with _aligned_malloc must be freed with _aligned_free (not std::free).

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