init: skip block verification for snapshot-sourced chains

After LoadSnapshot, the daemon has headers + UTXOs but the raw block
bodies haven't been downloaded yet — they'll arrive via P2P as the
node syncs past the snapshot tip. LoadBlockIndex's verification
loop tries to read the last 50 block bodies from disk and fails
with 'OpenBlockFile failed' because the data isn't on disk yet.

Add fLoadedFromSnapshot global, set true at the end of successful
LoadSnapshot. In both txdb-leveldb.cpp and txdb-rocksdb.cpp LoadBlockIndex
verification loops, when ReadFromDisk fails AND fLoadedFromSnapshot is
true, log a warning and continue (the UTXO set itself was already
content-hash verified during LoadSnapshot, so we have strong evidence
the chain state is correct).

For non-snapshot chains (full blk0001.dat downloaded, normal IBD), the
ReadFromDisk failure remains a fatal error as before.

Combined with the prior fix in utxosnapshot.cpp that sets
fSerializeChainTrust=true before writes, the full snapshot path now
works end-to-end on a fresh datadir.

src/txdb-leveldb.cpp

@@ -624,7 +624,18 @@ bool CTxDB::LoadBlockIndex()
             break;
         CBlock block;
         if (!block.ReadFromDisk(pindex))
+        {
+            // Snapshot-sourced chains have block headers + UTXOs but not raw
+            // block bodies on disk yet. Skip verification for those — the
+            // UTXO set itself was content-hash verified during LoadSnapshot.
+            // For non-snapshot chains, this remains a fatal error.
+            if (fLoadedFromSnapshot) {
+                printf("LoadBlockIndex(): block %d not on disk (snapshot-sourced), skipping verification\n",
+                       pindex->nHeight);
+                continue;
+            }
             return error("LoadBlockIndex() : block.ReadFromDisk failed");
+        }
         if (nCheckLevel>0 && !block.CheckBlock(true, true, (nCheckLevel>6)))
         {
             printf("LoadBlockIndex() : *** found bad block at %d, hash=%s\n", pindex->nHeight, pindex->GetBlockHash().ToString().c_str());