Replace internal requires with Ruby autoload (#120)

[andrew2net]

Summary

Fixes #120 — the circular require between OrganizationType and Subdivision reported by downstream gems (metanorma-taste) under Ruby 4.0 with warnings:

lib/relaton/bib/model/organization_type.rb:16: circular require considered harmful - .../subdivision.rb

OrganizationType.included did require_relative "subdivision" inside the hook, while Subdivision opens with include OrganizationType — so loading subdivision.rb re-entered the hook and re-required the in-progress file.

Per @ronaldtse's direction on the issue, this replaces the internal require_relative chains with Ruby autoload via a central manifest, which makes load order irrelevant and removes this whole class of ordering bugs.

Changes

• New lib/relaton/bib/model_autoload.rb — central autoload manifest for every Relaton::Bib model + Converter constant. Also eagerly requires lutaml/model/lutaml/xml and runs the Lutaml::Model::Config.configure { xml_adapter_type = :nokogiri } block (moved out of item.rb) so serialization works regardless of which class is referenced first.
• item.rb — dropped the ~39-line require_relative block and the class Relation < Serializable; end forward stub.
• relation.rb — now self-contained (class Relation < Lutaml::Model::Serializable), autoloaded.
• organization_type.rb — deleted the in-hook require_relative "subdivision" (the core fix).
• Dropped sibling require_relative from contact, ext, organization, date, series, bibitem, bibdata, bibitem_shared, bibdata_shared.
• Kept require_relative for converter method-partials (reopens, which autoload cannot express).
• Documented the new convention in CLAUDE.md.

Why it resolves the cycle

In both include orders, class Subdivision < Serializable / class Organization < Serializable opens its own constant before the include OrganizationType that fires the hook referencing Subdivision — so the self-reference resolves to an already-open constant instead of re-triggering the load.

Tests

New spec/relaton/bib/autoload_spec.rb:

• structural (no in-hook require), self-contained Relation superclass, and nokogiri-adapter guards;
• functional round-trips (organization with nested subdivision; standalone subdivision);
• load-order-independence subprocesses (Subdivision-first and Organization-first);
• a -W2 subprocess guard asserting no relaton-bib circular require warning (scoped so unrelated third-party warnings don't false-fail).

Full suite: 229 examples, 0 failures. RuboCop clean on new files.