test: cover cluster limits during reorgs #36214

This PR adds functional test coverage for cluster-limit handling during reorgs.

Transactions resurrected from disconnected blocks are added before their existing mempool children are linked in the transaction graph. Once those dependencies are restored, oversized clusters must be trimmed back within their configured count and size limits.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36214.

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Possible typos and grammar issues:

• the i'th one spending the (i-1)'th -> the i-th one spending the (i-1)-th [ordinal typo]

^{2026-09-10 04:11:16}

cc @instagibbs @sipa

I think the only thing this test adds over test_chainlimits_exceeded and transaction_graph_test is testing the pre-existing mempool children are added back in to the clusters. Thoughts?

I think the only thing this test adds over test_chainlimits_exceeded and transaction_graph_test is testing the pre-existing mempool children are added back in to the clusters.

Agree, transaction_graph_test covers reconnecting existing children. These tests cover clusters that become oversized only after reconnecting existing children, exercising Trim() for count, size, and merged clusters.