Add JsonPath module with AST parser and evaluator

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Add a new Maven module implementing JsonPath (Goessner)

Goal

• Add a new Maven module (Java 21) that implements JsonPath as described by Stefan Goessner.
• No runtime dependencies outside java.base (and the existing json-java21 core module).
• Parse JsonPath strings into a custom AST.
• Evaluate JsonPath AST against a JSON document parsed by the existing jdk.sandbox.java.util.json API.
• Do NOT parse JSON strings in JsonPath (core library already handles parsing).

Requirements

• Pure Java 21, functional/data-oriented style (records + sealed interfaces).
• No external libraries at runtime.
• TDD: add unit tests for every example on https://goessner.net/articles/JsonPath/index.html and implement incrementally.
• Keep behavior aligned with the article examples (selectors, recursion, wildcards, unions, slices, filters).

Acceptance

• New module builds in the multi-module Maven build.
• Tests cover all article examples.
• Public API includes a JsonPath.parse(String) (or equivalent) returning an AST-backed compiled expression and an evaluator over JsonValue.