fix(messagequeue): remainder-aware fair share and orphan sweep

[behinddwalls]

Summary

Why?

The MySQL queue subscriber capped every node at ceil(P/N) partitions, computed independently per node. The caps sum to more than P, so uneven splits can settle into stable starvation: with 12 partitions and 5 subscribers every cap is 3 (sum 15), and 3/3/3/3/0 is a legal end state where every owner sits "at cap" and nothing ever obliges anyone to shed for the empty subscriber. Separately, nothing guaranteed an unleased partition would eventually be picked up if the share arithmetic misfired (divergent heartbeat views during the staleness window, or a subscriber that heartbeats without ever acquiring), and every shedding rebalance tick logged a spurious lease renewal failed: ErrLeaseExpired because rebalance sorted the shared slice in place and renewal then ran over the just-released tail.

What?

• fairShareCap is now remainder-aware: each subscriber ranks itself in the sorted ActiveSubscribers list; the first P mod N ranks get floor(P/N)+1 and the rest floor(P/N), so per-rank caps sum to exactly P. A starved subscriber or an unclaimed partition now implies a peer over/under its cap that rebalance and discovery resolve — neither is a stable state. The signature, the 0 = unlimited contract, and the minimum-1 floor are unchanged; a subscriber missing from its own active view falls back to ceil over N+1 contenders (never unlimited, never zero).
• Orphan sweep: every 2 x LeaseDurationMs the discovery tick runs one uncapped acquisition pass. TryAcquireLease cannot steal a valid lease, so the sweep is a no-op in a healthy group, but a partition left unleased for any reason is picked up by whichever subscriber sweeps first; the next rebalance sheds any over-cap grab once a peer has capacity.
• rebalance sheds from a sorted copy instead of reordering the caller's slice, and returns the released partitions; the lease tick renews only the kept set, eliminating the spurious ErrLeaseExpired error log on every shedding tick.

Test Plan

• ✅ make test — new TestSubscriber_FairShareCap covers the 12/5 starvation case (caps 3,3,2,2,2), P<N flooring, the missing-heartbeat fallback, and a property subtest asserting per-rank caps sum to exactly P for all N≤6, P≤13; TestSubscriber_Rebalance* cover the released-tail contract and that the input slice is not reordered.
• ✅ bazel test //test/integration/extension/messagequeue/... — full Docker suite, including new TestRebalance_NoStarvation_UnevenSplit (12 partitions / 5 subscribers converge with every subscriber owning 2–3, total 12) and TestRebalance_OrphanSweep (phantom heartbeats cap the real subscriber at 1 of 3 partitions; all 3 messages are still delivered and acked via the sweep).