fix: remove global xml_adapter_type override

[ronaldtse]

Problem

lib/unitsml.rb unconditionally sets the global Lutaml::Model::Config.xml_adapter_type = :ox at require time:

Lutaml::Model::Config.configure do |config|
  config.xml_adapter_type = RUBY_ENGINE == "opal" ? :oga : :ox
end

This runs when require "unitsml" is called (e.g. via plurimath → unitsml), silently overwriting the global adapter for every other gem in the same Ruby process.

Impact

• Gems like rfcxml that explicitly set :nokogiri have their configuration overridden.
• In metanorma-ietf, this caused to_xml to hang on repeated calls because the adapter was switched mid-process from :nokogiri to :ox.
• The :ox adapter has known reentrancy issues that manifest as hangs when used in a process that was initialized with a different adapter.

Reproduction

require "lutaml/model"
Lutaml::Model::Config.xml_adapter_type = :nokogiri
puts Lutaml::Model::Config.xml_adapter_type  # => :nokogiri

require "unitsml"
puts Lutaml::Model::Config.xml_adapter_type  # => :ox  (silently changed!)

Fix

Two changes:

1. Delete the global configure block from lib/unitsml.rb.
2. Add ox as a runtime dependency in the gemspec.

Why this works — the adapter convention for lutaml-model gems

Lutaml::Model::Config.xml_adapter_type is a global singleton — one per process. When multiple gems set it, the last require wins, which is fragile and order-dependent. The correct convention is:

Only the application boundary sets the adapter. Libraries never do.

Stack:  metanorma-ietf (app)  →  rfcxml (lib)  →  lutaml-model
              ✅ sets adapter     ❌ must NOT       auto-detects
                                   set adapter

Stack:  unitsml CLI (app)   →  unitsml (lib)  →  lutaml-model
              ✅ sets adapter     ❌ must NOT       auto-detects
                                   set adapter

How auto-detection handles the default case

When require "lutaml/model" runs, it auto-detects the best available adapter (nokogiri > ox > oga):

# lutaml-model lib/lutaml/model/xml.rb:29-31
if (adapter = Lutaml::Model::Xml.detect_xml_adapter)
  Lutaml::Model::Config.xml_adapter_type = adapter
end

This is already the right default. The problem is libraries overriding it afterward.

What each gem must do

Gem type	Responsibility
Library gems (unitsml, rfcxml, etc.)	Declare adapter gem in gemspec (e.g. spec.add_dependency "ox"). Do NOT call Config.xml_adapter_type =. Auto-detection finds the installed adapter.
Application gems (metanorma-ietf)	Set Config.xml_adapter_type in application init, before loading libraries. This sticks because libraries no longer override it.
CLI entry points	Set Config.xml_adapter_type in exe/xxx before require "unitsml".
Tests	Already correct — spec_helper.rb sets the adapter. Tests are an application boundary.

Why this is seamless

Scenario	What happens
gem install unitsml standalone	gemspec pulls in ox → auto-detection finds ox → works
metanorma-ietf sets :nokogiri then loads unitsml	unitsml does not override → :nokogiri sticks → works
Auto-detection only	picks best available adapter → works
Multiple adapters installed	application/CLI picks one → works

Other gems that should adopt this convention

• rfcxml: same fix — remove Config.xml_adapter_type = :nokogiri from lib/rfcxml.rb, add nokogiri as gemspec dependency.
• Any other library using lutaml-model: same pattern — never call Config.xml_adapter_type = in library code, declare adapter gem in gemspec.