UTXO snapshot support, script verify cache, and Tor sync tuning (v5.8.7)

- Add UTXO snapshot dump/load system (utxosnapshot.cpp/h) for fast initial sync
- Add dumputxoset RPC command to create snapshots from current chain state
- Add script verification cache (sigcache.h) to skip re-verifying scripts
  already validated during mempool acceptance
- Bootstrap: try UTXO snapshot first (fast path), fall back to full bootstrap
- Support manual utxo-snapshot.bin loading on startup
- Tune sync parameters for Tor: increase timeouts, reduce buffer sizes
- Header sync cache: TTL-based eviction instead of full cache clear
- Reduce orphan block limits and script check batch size for lower memory usage

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/trianglesrpc.cpp

@@ -319,6 +319,7 @@ static const CRPCCommand vRPCCommands[] =
     { "invalidateblock",        &invalidateblock,        false,  false },
     { "reconsiderblock",        &reconsiderblock,        false,  false },
     { "recalculatesupply",      &recalculatesupply,      false,  false },
+    { "dumputxoset",            &dumputxoset,            false,  false },
     { "reservebalance",         &reservebalance,         false,  true},
     { "checkwallet",            &checkwallet,            false,  true},
     { "repairwallet",           &repairwallet,           false,  true},