Function canonicalizeSchemaForFingerprint

Canonicalization contract for schemaFingerprint:

1. Normalize $defs → definitions (Graviola convention).
2. Recursively sort object keys lexicographically at every level.
3. Preserve array element order (required, enum, allOf order are semantic).
4. Exclude top-level version and $schema from the hashed payload.
5. $ref strings are normalized only indirectly via defs→definitions rename; callers should use consistent ref targets before fingerprinting.

The fingerprint is stable across property order permutations on the same schema.