Merge bitcoin#35670: net: optimize compact block extra tx iteration

1a3cbf1 net: optimize compact block extra tx iteration (Lőrinc)

Pull request description:

  **Problem:** `vExtraTxnForCompact` gives compact block reconstruction one more source for recently removed transactions.
  Before this PR, the first insertion resized the cache to its configured capacity, so before the cache was full, `PartiallyDownloadedBlock::InitData()` also scanned default `{Wtxid::ZERO, nullptr}` entries that had never been added to the cache.
  Those unused entries could still participate in reconstruction short-id matching.

  **Fix:** Reserve the configured capacity and append entries until the cache is full.
  Once full, keep the same ring-buffer overwrite behavior, configured maximum, and replacement order.

  **Risk:** Triggering the affected duplicate-match branch requires a block-specific 6-byte short-id collision while the extra-txn cache still contains default null slots.
  With a 16-thread benchmark of the `CBlockHeaderAndShortTxIDs` nonce-grinding path, the 50% collision time was ~178 days on my machine.

ACKs for top commit:
  davidgumberg:
    crACK bitcoin@1a3cbf1
  darosior:
    utACK 1a3cbf1
  w0xlt:
    ACK 1a3cbf1
  sedited:
    ACK 1a3cbf1

Tree-SHA512: 9022a2ea5f3ff4279bdee7fa0316d6e5c922be3f7c566743b4d16708c49c0c8dc8a8dcbd60530a0128277db6f192355be88312419952213ae06353a0d937c507

Author: sedited

src/net_processing.cpp

@@ -1908,11 +1908,13 @@ std::vector<CTransactionRef> PeerManagerImpl::AbortPrivateBroadcast(const uint25
 
 void PeerManagerImpl::AddToCompactExtraTransactions(const CTransactionRef& tx)
 {
-    if (m_opts.max_extra_txs <= 0)
-        return;
-    if (!vExtraTxnForCompact.size())
-        vExtraTxnForCompact.resize(m_opts.max_extra_txs);
-    vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+    if (m_opts.max_extra_txs == 0) return;
+    if (vExtraTxnForCompact.size() < m_opts.max_extra_txs) {
+        if (vExtraTxnForCompact.empty()) vExtraTxnForCompact.reserve(m_opts.max_extra_txs);
+        vExtraTxnForCompact.emplace_back(tx->GetWitnessHash(), tx);
+    } else {
+        vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+    }
     vExtraTxnForCompactIt = (vExtraTxnForCompactIt + 1) % m_opts.max_extra_txs;
 }