feat: recognise type=none as null-nodestore config

NullFactory (type=none) already provides the exact null-backend
semantics: fetchNodeObject returns notFound, store is a no-op, no disk
I/O. Previously SHAMapStoreImp treated any non-"rwdb" type as
disk-backed and called dbPaths() unconditionally, crashing with
boost::filesystem::create_directories on an empty path.

- Recognise "none" alongside "rwdb" as a memory backend (skips
  dbPaths() and takes the memory-backend rotation path).
- On type=none, set XAHAU_RWDB_NULL=1 (overwrite=0) so the existing
  isRWDBNullMode() helpers in SHAMapSync, InboundLedger, Ledger etc.
  detect null-mode semantics (FBC liveness+anchor, setFullyWired,
  rotation-copy skip) without requiring the env var to be set
  separately.

Makes type=none a first-class null-backend config declaration,
equivalent to type=rwdb + XAHAU_RWDB_NULL=1 but without the env-var
dance. Users can now write:

  [node_db]
  type = none
  online_delete = 16

src/xrpld/app/misc/SHAMapStoreImp.cpp

@@ -154,7 +154,17 @@ SHAMapStoreImp::SHAMapStoreImp(
     }
 
     get_if_exists(section, "online_delete", deleteInterval_);
-    isMemoryBackend_ = boost::iequals(get(section, "type"), "rwdb");
+    auto const backendType = get(section, "type");
+    isMemoryBackend_ = boost::iequals(backendType, "rwdb") ||
+                       boost::iequals(backendType, "none");
+
+    // type=none is the declared null-nodestore config (via NullFactory).
+    // Propagate to XAHAU_RWDB_NULL so isRWDBNullMode() in other components
+    // (SHAMapSync, InboundLedger, Ledger) picks up null-mode semantics via
+    // their file-local helpers. overwrite=0 preserves any value the user
+    // has already set.
+    if (boost::iequals(backendType, "none"))
+        ::setenv("XAHAU_RWDB_NULL", "1", 0);
 
     // For RWDB, default online_delete to ledger_history only if user did not
     // explicitly set online_delete.  Clamp to the minimum so an implicit