ANTLR-based XPathMatcher implementation

[knutwannheden]

Replace regex-based XPath parsing with a proper ANTLR grammar for improved correctness, maintainability, and performance.

Key Changes

• ANTLR Grammar: New XPathLexer.g4 and XPathParser.g4 provide a formal grammar for the supported XPath subset
• XPathCompiler: Compiles XPath expressions into an intermediate representation at match time (lazy DCL pattern)
• Bottom-up Matching: Walk the cursor chain instead of building path arrays - more efficient for deep trees
• Allocation-free Evaluation: Unified expression evaluation avoids allocations during matching
• Fail-fast Validation: Unsupported XPath functions throw at parse time rather than silently failing at match time

Supported XPath Features

• Absolute/relative paths: /root/child, child/grandchild
• Descendant-or-self: //element
• Wildcards: /root/*
• Attributes: /@attr, /@*
• Predicates: [@attr='value'], [child='value'], [1], [last()]
• Functions: local-name(), contains(), starts-with(), text(), position(), last(), not()
• Logical operators: and, or
• Axes: parent::, self::, .., .

Test Improvements

Tests now verify exact match counts instead of just boolean existence, providing stronger validation of XPath matching behavior.