Document the public API

[samuel-williams-shopify]

Summary

• document all previously undocumented public classes, modules, and methods
• add typed parameter, return value, yielded block, and exception pragmas following the Decode Ruby documentation guidelines
• mark the XRB and generated fallback parser modules as namespaces

Verification

• bundle exec bake decode:index:coverage lib — 121/121 public definitions documented
• bundle exec bake test — 133 tests, 153 assertions
• XRB_PREFER_FALLBACK=1 bundle exec bake test — 133 tests, 153 assertions
• bundle exec rubocop — 50 files, no offenses
• bundle exec bake xrb:parsers:generate_fallback — generated parser remains reproducible
• git diff --check