Remove stale ox dependency (unused in code; hijacks moxml adapter auto-detection)

[ronaldtse]

Summary

unitsml.gemspec declares spec.add_dependency "ox", but no code in lib/ references Ox at all (no require "ox", no Ox. constant use) — the dependency appears to be stale from before the moxml/lutaml-model migration.

Why this matters beyond hygiene

The stale dependency has an active side effect through moxml's adapter auto-detection. Moxml::Config.detect_loaded_adapter prefers :ox whenever the Ox constant is defined:

def detect_loaded_adapter
  return :nokogiri if Object.const_defined?(:Nokogiri)
  return :ox if Object.const_defined?(:Ox)   # <- any loaded ox gem hijacks this
  return :oga if Object.const_defined?(:Oga)
  nil
end

Because unitsml drags ox into every consumer's bundle (and sibling gems like plurimath load it), all moxml consumers in the process — unitsml's own lutaml-model serialization included — silently run on the ox adapter even when the application never chose ox.

On TruffleRuby, ox is currently a crash hazard under sustained cext load (non-deterministic "dead handle" failures, truffleruby#4454), which we hit packaging Metanorma with tebako. Removing the unused dependency lets applications pick their XML backend deliberately (e.g. leptris via moxml ≥ 0.5, or nokogiri) instead of having ox forced on them by a gem that never calls it.

Proposed change

Remove spec.add_dependency "ox" from the gemspec (no code change needed — nothing references it). Happy to send the one-line PR if you agree.