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/chain.h

@@ -399,7 +399,16 @@ public:
 
     //! (memory only) Sequential id assigned to distinguish order in which blocks are received.
     uint32_t nSequenceId;
-    
+
+    //! (memory only) Set true once this block's RandomX PoW has been verified by the parallel
+    //! pre-verification pool, letting the inline check in CheckBlockHeader skip the recompute.
+    //! Written by exactly one pre-verify worker (1:1 with the block) and read by the connect
+    //! thread only AFTER the pool barrier (CCheckQueue::Wait provides the happens-before), so a
+    //! plain bool is race-free here. NOT serialized — a pure optimization hint; the inline
+    //! CheckRandomXSolution remains the consensus authority. (Plain bool, not std::atomic, so
+    //! CBlockIndex stays copyable for CDiskBlockIndex's `CBlockIndex(*pindex)` construction.)
+    bool fRandomXVerified;
+
     void SetNull()
     {
         phashBlock = NULL;
@@ -414,6 +423,7 @@ public:
         chainPower = CChainPower();
         nTx = 0;
         nChainTx = 0;
+        fRandomXVerified = false;
 
         // Shieldex Index chain stats
         nChainPayments = 0;