Add the DOM/tree editing primitives from the audit that the editor use
case needs:
- Attribute helpers: attributeValue(_:), setAttribute(_:_:) (update in
place or append), removeAttribute(_:).
- shallowCopy() copies a node alone, complementing the deep copy().
- precedes(_:) compares two nodes in document order (ancestor before
descendant, earlier sibling before later; unrelated nodes compare false).
- normalize() coalesces adjacent text children, drops empty text, and
recurses into elements; CDATA is left intact.
First-class namespace/DOCTYPE/entity-reference nodes (an invasive
TreeNodeKind change) and cross-document adoption/ownerDocument (no
document-identity model) are left for a follow-up.
Closes#90
