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/chainparams.h

@@ -69,6 +69,17 @@ public:
         double fTransactionsPerDay;
     };
 
+    /** Trusted UTXO-snapshot (assumeutxo-style) anchor. When `hash` is set, a node loading a
+     *  snapshot via -loadutxosnapshot must produce exactly this content hash at this height,
+     *  otherwise the snapshot is refused. Null hash = not configured (loading requires the
+     *  explicit -loadutxosnapshotunsafe override, e.g. for regtest/testing). Mirrors the
+     *  hardcoded-checkpoint trust model. */
+    struct AssumeutxoData {
+        int height;
+        uint256 hash;
+        bool IsNull() const { return hash.IsNull(); }
+    };
+
     enum Bech32Type {
         SAPLING_PAYMENT_ADDRESS,
         SAPLING_FULL_VIEWING_KEY,
@@ -105,6 +116,7 @@ public:
     const std::string& Bech32HRP(Bech32Type type) const { return bech32HRPs[type]; }
     const std::vector<uint8_t>& FixedSeeds() const { return vFixedSeeds; }
     const CCheckpointData& Checkpoints() const { return checkpointData; }
+    const AssumeutxoData& Assumeutxo() const { return assumeutxoData; }
     /** Return the founder's reward address and script for a given block height */
     std::string GetFoundersRewardAddressAtHeight(int height) const;
     CScript GetFoundersRewardScriptAtHeight(int height) const;
@@ -144,6 +156,7 @@ protected:
     bool fMineBlocksOnDemand = false;
     bool fTestnetToBeDeprecatedFieldRPC = false;
     CCheckpointData checkpointData;
+    AssumeutxoData assumeutxoData; // null by default; set per-network in chainparams.cpp once a snapshot hash is published
     std::vector<std::string> vFoundersRewardAddress;
 };