nx_kv_arena_test.nx

nx_kv_arena_test.nx -- smoke for PagedAttention page pool. Exercises: 1. Allocation + canary + initial state (all pages free) 2. Sequence creation + page appends from empty pool 3. Pool exhaustion: more requests than pages -> POOL_EXHAUSTED 4. Sequence free returns pages to the pool 5. Newly-free pages can be allocated by a different sequence 6. Multi-sequence independence (each owns its own indices) 7. Page byte-pointer arithmetic: indices map to distinct byte offsets in page_storage 8. Canary tamper on arena and sequence 9. Bad-input gates 10. Sealed-enum verdict gate 11. Idempotent re-free (calling free twice on same seq is no-op)