IBD/sync speedups: parallel RandomX pre-verify, adaptive dbcache, P2P download fixes

- Parallel RandomX PoW pre-verification pool (CCheckQueue) run ahead of the serial
  connect; consensus-neutral (inline CheckRandomXSolution fallback still verifies
  anything not pre-verified). New -randomxverifythreads (default = -par).
- Adaptive dbcache: default sizes the UTXO/coins cache to most of RAM and shrinks
  under memory pressure, always leaving a reserve free; -dbcache pins a fixed value.
- P2P block download: bounded socket recv-drain loop (tlsmanager); frontier-block
  reassignment to break head-of-line stalls (-blockreassigntimeout); ProcessGetData
  serves a bounded batch of blocks per pass instead of one (fixes the serve-side
  one-block-per-tick throttle that caps download network-wide).
- assumeutxo: dumptxoutset RPC + LoadSnapshot machinery + AssumeutxoData chainparams.
- Signed bootstrap verification (util/bootstrap-dragonx.sh, util/sign-bootstrap.md).
- gtest: RandomX pre-verify consensus-equivalence test + UTXO-snapshot round-trip;
  revived the gtest harness (Makefile.am include fix, Makefile.gtest.include).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main.h

@@ -100,7 +100,11 @@ static const int MAX_BLOCKS_IN_TRANSIT_PER_PEER = 16;
 /** Timeout in seconds during which a peer must stall block download progress before being disconnected. */
 static const unsigned int BLOCK_STALLING_TIMEOUT = 2;
 /** Number of headers sent in one getheaders result. We rely on the assumption that if a peer sends
- *  less than this number, we reached its tip. Changing this value is a protocol upgrade. */
+ *  less than this number, we reached its tip. Changing this value is a protocol upgrade: the
+ *  continuation logic (main.cpp, "nCount == MAX_HEADERS_RESULTS") and the serve-side limit must
+ *  match across the network, so a single node raising it unilaterally would mis-detect a stock
+ *  peer's 160-header reply as "tip reached" and stall header sync. Raise only as a coordinated
+ *  network upgrade (with a protocol-version bump). */
 static const unsigned int MAX_HEADERS_RESULTS = 160;
 /** Size of the "block download window": how far ahead of our current height do we fetch?
  *  Larger windows tolerate larger download speed differences between peer, but increase the potential
@@ -155,6 +159,7 @@ extern bool fExperimentalMode;
 extern bool fImporting;
 extern bool fReindex;
 extern int nScriptCheckThreads;
+extern int nRandomXVerifyThreads;
 extern bool fTxIndex;
 extern bool fZindex;
 extern bool fIsBareMultisigStd;
@@ -930,6 +935,19 @@ extern CChain chainActive;
 /** Global variable that points to the active CCoinsView (protected by cs_main) */
 extern CCoinsViewCache *pcoinsTip;
 
+/** Global variable that points to the coins database (chainstate/, protected by cs_main).
+ *  Exposed for the UTXO-snapshot (assumeutxo-style) dump/load paths. */
+class CCoinsViewDB;
+extern CCoinsViewDB *pcoinsdbview;
+
+/** Activate a trusted UTXO snapshot (already written to the chainstate DB by LoadSnapshot) as the
+ *  chain tip at its height H, without replaying blocks 0..H. Headers for H must already exist. */
+struct CUTXOSnapshotHeader;
+bool LoadSnapshotChainstate(const CUTXOSnapshotHeader& header, std::string& strError);
+/** Height H of a loaded UTXO snapshot (assumeutxo). Reorgs whose fork point is below H are refused
+ *  because the node has no block/undo data for 0..H. -1 means no snapshot is in effect. */
+extern int nAssumeutxoSnapshotHeight;
+
 /** Global variable that points to the active block tree (protected by cs_main) */
 extern CBlockTreeDB *pblocktree;