fix: match fixed capacity to the allocated storage

[feritcemkahraman]

A fixed queue created with capacity 1.5 allocates one slot, but keeps 1.5 as its capacity. After the first push, the overflow check never matches; another push can lose the queued item. A capacity of 0.5 similarly allocates no storage yet accepts a push.

Use the allocated ID array length as the fixed capacity, matching the typed array constructor's length conversion. Dynamic queues keep their infinite capacity.

Validation: both new regression tests failed before the fix. npm test passes ESLint, TypeScript declaration generation and all 7 tests afterward (Node 24). git diff --check passes.