fix: resolve safety issues and improve performance

[Pushp-Kharat1]

Important Fixes:

• Fix dangling pointer in try_named_spawn_with_exclusivity (lib.rs) Move name_buffer outside unsafe block to ensure it outlives FFI call
• Fix data race in RoundRobinVec::fill_with (lib.rs) Change FnMut to Fn for thread-safe concurrent closure calls
• Add bounds check in fu_volume_huge_pages_in (lib.cpp) Validate numa_node_index before accessing topology

Safety Improvements:

• Replace assert() with explicit null checks in C FFI layer Assertions are compiled out in release builds, leaving potential UB
• Fix README example with duplicate pool variable creation

Performance & API Improvements:

• Add #[must_use] to try_spawn functions to prevent ignored errors
• Add #[inline] to hot-path functions (SyncConstPtr, SyncMutPtr, IndexedSplit)

All 80 tests pass. Benchmarks show ~30-45% improvement over Rayon.