Merge origin/master (Krystie's RocksDB+IBD integration + snapshot wiring)

Reconciles two parallel implementations of multi-backend chain DB:
local kept its MakeChainDB factory + std::filesystem + unconditional
RocksDB + abstracted utxosnapshot, since those are downstream of the
boost-cleanup, smessage-RocksDB-port, and CTxDBBase abstraction work.

Preserved from origin (Krystie's branch):
- Block 2,203,594 checkpoint and matching mapSnapshotHashes entry
  for P2P snapshot verification (src/checkpoints.cpp)
- Headers-first IBD stall-recovery path: during IBD, replace the
  legacy PushGetBlocks fallback with RequestHeaderSyncRefillAllPeers
  + QueueHeaderSyncBlocksParallel so a stall on a weak peer set
  doesn't park at a low common ancestor (src/main.cpp SendMessages)

Discarded from origin:
- src/txdb.cpp (CActiveTxDB wrapper) — superseded by txdb-factory.cpp
- BUILD_ROCKSDB-gated paths and inline LevelDB+RocksDB code in
  utxosnapshot.cpp — already factored out behind CTxDBBase
- Public ReadRawBytes/WriteRawBytes/... wrappers added to
  CTxDBBase for CActiveTxDB; no remaining callers
- GetActiveChainDbDirName / UseRocksDbBackend in bootstrap.cpp;
  switched to GetChainDataDir()

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/checkpoints.cpp

@@ -38,6 +38,7 @@ namespace Checkpoints
         {2200000, uint256("0x0a8d0442f031f1258120f713f34e45f4f9a625fb753558e27b89b32ad5a9a740")},
         {2205000, uint256("0xf7aa893ec012181e321783d6a5487addf6997377908faa3e760c9054a4217d29")},
         {2206000, uint256("0x780ae878f8b10b6cbd51ceb2c0799c90d3551fa916be62974375071b9e36581c")},
+        {2203594, uint256("0x5e016ae5d1f163c6679292b717a3db467a39d24b0a315182f4783caa79c722d8")},
         {2207000, uint256("0x8836d67b0f08036c4a7c26ff0a29d4461a52b6d8f552165ad9c1abec2f3cadfd")},
     };
 
@@ -50,7 +51,7 @@ namespace Checkpoints
     // here. The corresponding (height, blockhash) must already exist in
     // mapCheckpoints / mapCheckpointsTestnet.
     static std::map<int, uint256> mapSnapshotHashes = {
-        // {2186940, uint256("0x...sha256-of-utxo-snapshot.bin...")},
+        {2203594, uint256("0x49b35dd01659975c4a31954f37174c6e2e8878dd0723ab306ccecd991c80f79a")},
     };
 
     static std::map<int, uint256> mapSnapshotHashesTestnet = {