fix: assign VarBinScheme to code 5 without renumbering existing schemes

Move VarBinScheme to StringCode(5) to avoid renumbering existing schemes:
- UNCOMPRESSED_SCHEME: 0 (unchanged)
- DICT_SCHEME: 1 (unchanged)
- FSST_SCHEME: 2 (unchanged)
- CONSTANT_SCHEME: 3 (unchanged)
- SPARSE_SCHEME: 4 (unchanged)
- VARBIN_SCHEME: 5 (new)

This preserves backward compatibility with existing compressed data.

Signed-off-by: Joe Isaacs <[email protected]>
Co-Authored-By: Claude <[email protected]>

Author: joseph-isaacs

vortex-btrblocks/src/string.rs

@@ -137,12 +137,12 @@ pub struct NullDominated;
 pub struct StringCode(u8);
 
 const UNCOMPRESSED_SCHEME: StringCode = StringCode(0);
-const VARBIN_SCHEME: StringCode = StringCode(1);
-const DICT_SCHEME: StringCode = StringCode(2);
-const FSST_SCHEME: StringCode = StringCode(3);
-const CONSTANT_SCHEME: StringCode = StringCode(4);
+const DICT_SCHEME: StringCode = StringCode(1);
+const FSST_SCHEME: StringCode = StringCode(2);
+const CONSTANT_SCHEME: StringCode = StringCode(3);
 
-const SPARSE_SCHEME: StringCode = StringCode(5);
+const SPARSE_SCHEME: StringCode = StringCode(4);
+const VARBIN_SCHEME: StringCode = StringCode(5);
 
 impl Scheme for UncompressedScheme {
     type StatsType = StringStats;