Phase 7.1+7.3: Cache-line placement of bytes_until_sample_ + lazy provider overhead validation

[jayakasadev]

Summary

Phase 7.1 — Hoist the per-thread Poisson sampler hot counter into its own cache line.

• SNMALLOC_CACHE_LINE_SIZE macro (128 on Apple Silicon via __APPLE__ + __aarch64__, 64 elsewhere)
• New Sampler::SamplerHotState nested struct, alignas(SNMALLOC_CACHE_LINE_SIZE), with int64_t bytes_until_sample at offset 0
• First non-static member of Sampler → TLS sampler's hot counter occupies its own cache line, no false-sharing with concurrent dealloc clears on adjacent fields
• SNMALLOC_LIKELY already present on the fast-path branch (annotated for clarity)

Phase 7.3 — Compile-time + runtime validation that lazy ClientMetaDataProvider adds zero overhead when profiling inactive.

• New test src/test/func/profile_overhead/profile_overhead.cc (auto-discovered by CMake subdirlist)
• static_asserts:
  • sizeof(LazyArrayClientMetaDataProvider<T>::StorageType) == sizeof(void*)
  • sizeof(NoClientMetaDataProvider::StorageType) == sizeof(Empty)
  • sizeof(snmalloc::Config::PagemapEntry) == sizeof(StandardConfigClientMeta<NoClientMetaDataProvider>::PagemapEntry) — proves lazy provider compiled in but NOT instantiated into default config's metadata
  • Sampler::kBytesUntilSampleOffset == 0
• Runtime: 1M × 32B alloc microbench, profile-off vs profile-on-but-inactive; asserts < 5% overhead ratio

Test plan

• func-profile_overhead-fast/check (SNMALLOC_PROFILE=ON)
• func-profile_overhead-fast (SNMALLOC_PROFILE=OFF, layout-only smoke)
• All 14 func-profile_* tests
• func-memory, func-statistics, func-first_operation, func-teardown, func-bits, func-memory_usage, func-external_pointer, func-large_alloc
• cargo test --release (snmalloc-rs, no features + --features profiling)