Avoid a temporary array when dequeuing groups

[OskarEichler]

Summary

Use Array#delete_at when dequeuing one group instead of creating a one-element slice and extracting its first item. Group selection and break decisions are unchanged.

Verification

• rbenv Ruby 4.0.6: existing rake test, 35 tests / 54 assertions, passes before/after.
• External deterministic queue corpus: 28,512 ordering/flag checks; 2,000 generated formatter outputs remain byte-identical.
• 10,000 dequeues: 20,000 → 10,000 Ruby allocations. A 20-call formatting workload changes 72,522 → 72,502 allocations; the allocation benefit depends on the number of groups actually dequeued.
• Timings varied around 2ms for dequeue and 10ms for the formatter workload. No reliable speedup or application-throughput improvement is claimed.
• No repository tests added/modified under the contributor's task constraint. Other Ruby engines/versions were not executed locally.

Breaking-change notes

None expected. delete_at(i) returns the same selected group as slice!(i, 1).first for the valid indices produced by this loop.