Implement RFC 6901 JSON Pointer as json-java21-jsonpointer module

[Copilot]

Adds a new json-java21-jsonpointer module implementing RFC 6901 JSON Pointer — the exact-address path syntax used by JSON Patch. Single runtime dependency: sibling json-java21.

What changed

New module: json-java21-jsonpointer

• JsonPointer — parse(), resolve(), exists(), tokens(), toString()
• JsonPointerSyntaxException — malformed pointer (no leading /, invalid ~ escape)
• JsonPointerResolutionException — pointer valid but target absent (missing key, OOB index, non-numeric array token, traversal into primitive)
• JsonPointerLoggingConfig — standard JUL logging base for tests, matching sibling modules

RFC 6901 §5 example table — all 12 entries covered:

JsonValue doc = Json.parse("""
    {"foo":["bar","baz"],"":0,"a/b":1,"m~n":8}
    """);

JsonPointer.parse("").resolve(doc);       // whole document
JsonPointer.parse("/foo/0").resolve(doc); // "bar"
JsonPointer.parse("/").resolve(doc);      // 0  (empty-string key)
JsonPointer.parse("/a~1b").resolve(doc);  // 1  (~1 decodes to /)
JsonPointer.parse("/m~0n").resolve(doc);  // 8  (~0 decodes to ~)

// Escape ordering: ~01 → ~1 (not /)
JsonPointer.parse("/foo").exists(doc);    // true
JsonPointer.parse("/missing").exists(doc); // false

Key behaviours:

• Escape decoding order mandated by RFC: ~1→/ first, ~0→~ second (so ~01 → ~1)
• - token correctly not resolvable under RFC 6901 (RFC 6902 append sentinel only)
• Leading-zero array indices ("01") rejected per RFC 6901 §4
• parse(p).toString() always round-trips

Module structure mirrors json-java21-jsonpath and json-java21-jtd exactly — same pom.xml shape, compiler flags (-Xlint:all -Werror), package convention, and doc style (JEP 467 ///).

Why this change is needed

JSON Pointer is the path/address syntax used by JSON Patch (RFC 6902) and needed as a foundation for future patch support. It is also useful standalone for navigating JSON documents by exact address.

How were these changes tested

51 unit tests covering: all RFC §5 example table entries, escape ordering edge cases, tokens() / exists() / toString() accessors, syntax rejection, resolution failures (missing key, OOB, non-numeric token, primitive traversal, - token, leading-zero index), and nested document resolution via parameterized cases.

Checklist

• Code builds / passes tests
• New tests added if needed
• Update to use CODING_STYLE_LLM.md convensions
• Documentation updated if needed
• AGENTS.md updated if appropriate