[codex] Implement namespace support

[domenic]

Summary

• Add a dedicated namespace construct that installs Web IDL namespace objects on exposed globals.
• Reuse the existing operation, attribute, and constant generation paths with namespace-specific handling for regular namespace members, readonly attributes, and SameObject caching.
• Add namespace fixture coverage and generated snapshots for partial namespaces, overloads, constants, CEReactions, SameObject, and WebIDL2JSCallWithGlobal.
• Update the README support matrix to list namespaces and partial namespaces as supported.

Validation

• npm run update-snapshots
• npm test
• npm run lint