Add versioned storage methods for incremental export support: - GetChangesSinceExport: returns changes since a commit hash, with NeedsFullExport flag for invalid/GC'd commits - CommitExists: checks if a commit hash exists, supports short prefixes Also fixes dolt_diff syntax for embedded driver (from_ref, to_ref, table). Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>