…erf regression

Ensures assertions and expensive checks are disabled in release builds when using build_cc, addressing issue microsoft#202.

Enables WaitOnAddress support (using Futex) for build_cc on Linux, resolving the spinning behavior issue. Also defines SNMALLOC_HAS_LINUX_RANDOM_H and SNMALLOC_PLATFORM_HAS_GETENTROPY which are standard on modern Linux.

…ild_cc only

Reverts NDEBUG change that broke cmake builds. Also guards SNMALLOC_HAS_LINUX_FUTEX_H and related definitions with #[cfg(feature = 'build_cc')] to prevent build errors when using cmake (default).

Explicitly sets WINVER/_WIN32_WINNT to 0x0A00 (Win10) by default to enable VirtualAlloc2 and WaitOnAddress. Sets to 0x0603 (Win8.1) if win8compat feature is enabled. Matches snmalloc CMake behavior and ensures WaitOnAddress is available.

Adds 'usewait-on-address' to default features in Cargo.toml. This aligns the Rust build behavior with upstream CMake defaults, where SNMALLOC_ENABLE_WAIT_ON_ADDRESS is ON by default. Prevents silent performance regression (spinning vs futex) for users relying on default features.

Add a new workspace member `xtask` containing a CLI tool to compare performance between the `build_cc` and `build_cmake` backends. The tool automates building, running a contention benchmark, and parsing throughput results to verify performance equivalence.

Also includes the new benchmark example `bench_contention` which allocates and deallocates memory across a ring of threads to stress the allocator.

Updates build.rs to use compiler flags (-D) instead of CMake variables for WINVER and _WIN32_WINNT when using the cmake builder. This ensures these critical definitions reach the snmalloc C++ compiler, enabling WaitOnAddress (Futex) support on Windows and resolving a performance discrepancy where build_cmake was ~20% slower than build_cc.

Refactor the platform-specific configuration in build.rs by replacing
match blocks with if/else statements for better clarity. This change
consolidates Windows and Unix flag handling, fixes MSVC flag definitions,
and adds specific support for FreeBSD.

Update the build script to honor CARGO_CFG_TARGET_OS and OPT_LEVEL
environment variables. This ensures that snmalloc is compiled with
settings consistent with the rest of the Rust project, rather than
relying solely on the "debug" feature flag.