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>

README.md

@@ -27,6 +27,21 @@ the entire history of Hush transactions; depending on the speed of your
 computer and network connection, it will likely take a few hours at least, but
 some people report full nodes syncing in less than 1.5 hours.
 
+# Fastest way to sync (bootstrap)
+
+The quickest way to get a fully-synced node is the signed bootstrap snapshot, which
+installs a pre-built blockchain so you skip re-validating the whole chain from genesis:
+
+```sh
+# Stop dragonxd first if it is running, then:
+./util/bootstrap-dragonx.sh
+```
+
+The script preserves your `wallet.dat` and `DRAGONX.conf`, verifies the download's
+checksums and (once a release key is published) its cryptographic signature, then starts
+you near the chain tip. If you prefer to sync from the network instead, a larger
+`-dbcache` (e.g. `-dbcache=2048`) noticeably speeds up the initial block download.
+
 # Banned by GitHub
 
 In working on this release, Duke Leto was suspended from Github, which gave Hush developers