refactor: make arena vacant list explicit

[tisonkun]

Summary

• represent the intrusive vacant-list head and links as Option<SlotId> instead of overloading slots.len() as a sentinel
• rename next_vacant to vacant_head and document the slot ID lifetime directly
• preserve the existing allocation, slot reuse, and stable-ID behavior without changing the size of the head or vacant-link fields because SlotId uses a NonZeroUsize niche

Validation

• cargo x check
• cargo x test
• cargo x lint
• interleaved baseline/branch benchmarks for completion and event cancellation remained flat