Handle trailing channel metadata after items

[danmactough]

Resolves #262

Summary

• Re-run feed metadata extraction when the enclosing channel/feed closes so trailing channel fields are not missed
• Keep item/entry nodes out of metadata parsing to avoid leaking articles into meta
• Add a regression fixture and tests for trailing RSS metadata, including normalize: false
• Clarify in the README that consumers who care about trailing metadata should listen for the meta event through stream end

Testing

• Added focused regression coverage in test/trailing-meta.js
• Verified the full test suite passes, including lint and type checks