Add ancestors, descendants, path, and line_number methods to Node

[Copilot]

Implements four node navigation methods referenced in documentation but previously unimplemented.

Changes

Node class (lib/moxml/node.rb)

• ancestors - Returns NodeSet of all ancestor nodes from parent to document
• descendants - Returns NodeSet of all descendant nodes (recursive children)
• path - Returns XPath expression locating the node
• line_number - Returns source line number (adapter-dependent)

Adapter implementations (lib/moxml/adapter/*.rb)

• Nokogiri/LibXML: Delegate to native path and line/line_num methods
• Oga/REXML/Ox/HeadedOx: Custom XPath generation with sibling position handling; line_number returns nil

Usage

doc = context.parse(xml)
title = doc.at_xpath('//book/title')

title.ancestors          #=> NodeSet[<book>, <library>, document]
title.path               #=> "/library/book/title"
title.line_number        #=> 5 (or nil if unsupported)

section = doc.at_xpath('//section')
section.descendants      #=> NodeSet[<book>, <title>, <author>, text nodes...]

Tests

Added comprehensive test coverage in spec/moxml/node_spec.rb for all four methods.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add more node methods</issue_title>
<issue_description>Several methods were mentioned in the original readme file, but weren't implemented, and it seems like nobody missed them:

• ancestors
• descendants
• path
• line_number

I'll remove them from readme for now and will work on them in a separate PR.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add more node methods #24

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