Add XML diff, patch, and merge operations to etree

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What this task asks for

Add recursive XML diffing, patch generation and application, reverse patching, three-way merge, and diff summaries. Add nil-safe recursive element equality and document diff operations for elements, text, and attributes. Generate and apply XML patch documents using positional XPath selectors and the XML patch namespace. Reverse patches in reverse operation order while handling element, attribute, text, and replacement operations. Merge three document versions with configurable identity, conflict classification, and optional automatic resolution. Expose diff summaries, document convenience methods, and merge provenance in document metadata. Preserve documented operation value types, string forms, default diff options, and default merge options.