fix: update for Zig 0.16 ArrayList and allocator API changes

- Replace deprecated `initCapacity(allocator, 0)` with `.empty` for ArrayList
  compatibility on Zig 0.16+.
- Switch benchmarks from `page_allocator` to `smp_allocator` per updated
  allocator recommendations in newer Zig.
- Clarify version differences in documentation related to ArrayList and
  allocator changes.

These updates ensure compatibility across Zig 0.14 to 0.16+.