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>

util/bootstrap-dragonx.sh

@@ -13,6 +13,20 @@ BOOTSTRAP_FALLBACK_URL="https://bootstrap2.dragonx.is"
 BOOTSTRAP_FILE="DRAGONX.zip"
 CHAIN_NAME="DRAGONX"
 
+# DragonX bootstrap signing public key (PEM, openssl-compatible).
+# WHY: the .md5/.sha256 files are served from the same host as the archive, so they
+# only detect transmission corruption — a compromised bootstrap server could publish a
+# malicious archive with matching checksums. A detached signature verified against THIS
+# embedded public key (shipped in the repo, not downloaded) closes that gap: a bad server
+# cannot forge a signature without the maintainer's offline private key.
+#
+# ROLLOUT: until the maintainer embeds a real key here and publishes DRAGONX.zip.sig,
+# this stays as the placeholder and signature enforcement is skipped (with a loud warning),
+# so existing users are unaffected. Once a real key is pasted in, an unsigned/invalid
+# bootstrap is refused (fail-closed). See util/sign-bootstrap.md for the signing procedure.
+BOOTSTRAP_PUBKEY_PLACEHOLDER="REPLACE_WITH_DRAGONX_BOOTSTRAP_PUBLIC_KEY_PEM"
+BOOTSTRAP_PUBKEY="$BOOTSTRAP_PUBKEY_PLACEHOLDER"
+
 # Determine data directory
 if [[ "$OSTYPE" == "darwin"* ]]; then
     DATADIR="$HOME/Library/Application Support/Hush/$CHAIN_NAME"
@@ -139,6 +153,7 @@ download_from() {
     local outfile="$DATADIR/$BOOTSTRAP_FILE"
     local md5file="$DATADIR/${BOOTSTRAP_FILE}.md5"
     local sha256file="$DATADIR/${BOOTSTRAP_FILE}.sha256"
+    local sigfile="$DATADIR/${BOOTSTRAP_FILE}.sig"
 
     info "Downloading bootstrap from $base_url ..."
     info "This may take a while depending on your connection speed."
@@ -149,14 +164,50 @@ download_from() {
     info "Downloading checksums..."
     download_file "$base_url/${BOOTSTRAP_FILE}.md5" "$md5file" || return 1
     download_file "$base_url/${BOOTSTRAP_FILE}.sha256" "$sha256file" || return 1
+    # Detached signature is optional during rollout (non-fatal if absent); enforcement
+    # is decided in verify_signature() based on whether a real public key is embedded.
+    rm -f "$sigfile"
+    download_file "$base_url/${BOOTSTRAP_FILE}.sig" "$sigfile" || warn "No signature file at $base_url (${BOOTSTRAP_FILE}.sig)"
     return 0
 }
 
+# Verify the detached signature of the archive against the embedded release public key.
+# Fail-closed once a real key is configured; skip (with warning) while the placeholder is in place.
+verify_signature() {
+    local archive="$1"
+    local sigfile="$2"
+
+    if [[ "$BOOTSTRAP_PUBKEY" == "$BOOTSTRAP_PUBKEY_PLACEHOLDER" ]]; then
+        warn "Bootstrap signature verification is not yet configured (no maintainer key embedded)."
+        warn "Relying on TLS + checksum integrity only. See util/sign-bootstrap.md."
+        return 0
+    fi
+
+    if ! command -v openssl &>/dev/null; then
+        error "openssl is required to verify the bootstrap signature but was not found. Install openssl and retry."
+    fi
+    if [[ ! -s "$sigfile" ]]; then
+        error "Bootstrap signature (${BOOTSTRAP_FILE}.sig) is missing; refusing to use an unsigned bootstrap."
+    fi
+
+    local pubfile
+    pubfile=$(mktemp)
+    printf '%s\n' "$BOOTSTRAP_PUBKEY" > "$pubfile"
+    if openssl dgst -sha256 -verify "$pubfile" -signature "$sigfile" "$archive" >&2; then
+        rm -f "$pubfile"
+        info "Bootstrap signature verified against embedded DragonX release key."
+    else
+        rm -f "$pubfile"
+        error "Bootstrap signature verification FAILED — the archive is NOT signed by the DragonX release key. Aborting; do not use this bootstrap."
+    fi
+}
+
 # Download the bootstrap and verify checksums
 download_bootstrap() {
     local outfile="$DATADIR/$BOOTSTRAP_FILE"
     local md5file="$DATADIR/${BOOTSTRAP_FILE}.md5"
     local sha256file="$DATADIR/${BOOTSTRAP_FILE}.sha256"
+    local sigfile="$DATADIR/${BOOTSTRAP_FILE}.sig"
 
     if ! download_from "$BOOTSTRAP_BASE_URL"; then
         warn "Primary download failed, trying fallback $BOOTSTRAP_FALLBACK_URL ..."
@@ -187,8 +238,11 @@ download_bootstrap() {
         warn "sha256sum not found, skipping SHA256 verification."
     fi
 
-    # Clean up checksum files
-    rm -f "$md5file" "$sha256file"
+    # Verify the cryptographic signature (fail-closed once a release key is embedded).
+    verify_signature "$outfile" "$sigfile"
+
+    # Clean up checksum + signature files
+    rm -f "$md5file" "$sha256file" "$sigfile"
 
     echo "$outfile"
 }