Simplify fair distribution algorithm

[asg0451]

Simplify item distribution logic in PollCoalescer using a round-robin queue for fair allocation.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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[github-actions]

Criterion benchmarks delta (ms; lower is better)

Benchmark	PR (ms)	Master (ms)	Δ (ms)	Δ (%)
e2e_rpc/rpc_add_batch_200	0.619	0.627	-0.008	-1.3%
e2e_rpc/rpc_poll_batch_200	52.346	52.824	-0.478	-0.9%
e2e_rpc/rpc_remove_each_200	24.270	25.618	-1.348	-5.3%
e2e_rpc/rpc_stress_like_p2_a2_items_200	1209.325	1214.560	-5.235	-0.4%
storage_add/add_available_500	8.309	8.257	0.052	+0.6%
storage_poll/get_next_available_entries_500	10.444	10.368	0.076	+0.7%
storage_remove/remove_in_progress_500	20.524	20.265	0.259	+1.3%