rocksdb: apply T010 review fixes (H1/H2/H3/M4) + CF routing disabled

H1: init.cpp now detects crashed migrations (MIGRATION_INCOMPLETE marker)
    and retries instead of opening a partial RocksDB. Refuses to start if
    the marker persists after migration attempt.
H2: LevelDB ExistsRaw now returns false for keys deleted in the active
    batch, matching ReadRaw and the RocksDB backend. Fixes latent
    cross-backend consensus split in intra-batch spend checks.
H3: All RocksDB close paths now go through close_rocksdb() which
    destroys CF handles before deleting the DB. Fixes RocksDB assertion
    / UB on shutdown and version-reset.
M4: Migration marker write is now flushed + verified (refuses to start
    migration if marker can't be written).

CF routing permanently disabled: GetCF() always returns nullptr (default
column family). The read path (NewIterator, LoadBlockIndex) only iterates
the default CF, so writes routed to per-prefix CFs were invisible to scans.
This is why -chaindb=rocksdb compiled clean but was never runtime-valid.
Existing CF-enabled DBs still open (handles retained for cleanup) but no
routing occurs. CF-aware iteration is a future follow-up.

txdb-factory: RocksDB is now the default backend (was still leveldb).
Tests updated for RocksDB-as-default expectations.

From Claude's ROCKSDB-T010-REVIEW-2026-07-01 audit on E:\repos\triangles.

src/txdb-factory.cpp

@@ -23,7 +23,10 @@ enum class ChainDbKind { LevelDB, RocksDB };
 
 ChainDbKind ResolveChainDbKind()
 {
-    std::string s = GetArg("-chaindb", std::string("leveldb"));
+    // RocksDB is the default backend. LevelDB remains selectable with
+    // -chaindb=leveldb and is retained as the migration source and fallback;
+    // its removal is deferred to a later phase after live-chain validation.
+    std::string s = GetArg("-chaindb", std::string("rocksdb"));
     for (auto& c : s) c = std::tolower(static_cast<unsigned char>(c));
 
     if (s == "leveldb")