Order-preserving, reversible key codec for on-disk sorted maps/sets.

Unlike hash maps (which SHA-1-hash their keys), sorted collections store the
real key bytes so they can be recovered on read and compared by the engine's
unsigned lexicographic byte comparison (`Arrays.compareUnsigned`). The codec
must therefore be:

  1. reversible  - `decode-key (encode-key k)` == k
  2. order-preserving - `sign(compareUnsigned (encode a) (encode b))`
                        == `sign(compare a b)` for any two keys.

Every encoding carries a leading 1-byte type tag. The tag both identifies the
type on decode and establishes a total order across types, so heterogeneous
keys never throw.

Supported key types: string, keyword, boolean, char, long, double, UUID,
Instant and Date. Strings encode as their UTF-8 bytes (already code-point
ordered); keywords use a flag + namespace + name layout so they sort like
Clojure's default comparator (see `keyword->bytes`); numeric/temporal/UUID
keys use order-preserving big-endian encodings.

Bridges the XITDBSorted* wrapper types to the Java Read/WriteSortedMap.
Keys are encoded/decoded through `xitdb.util.sorted-key` (order-preserving,
reversible) rather than hashed, so the real key is recoverable on read.