Upgrade to track jdk.incubator.json

[simbo1905]

Goal

Upgrade this backport to track the upstream jdk.incubator.json module from the OpenJDK jdk-sandbox json branch.

Background

Upstream has moved the JSON API from java.util.json (in java.base) to jdk.incubator.json (new incubator module). See #144 for full context.

Required Changes

Public API (jdk.sandbox.java.util.json → stays same package)

• Method renames on JsonValue:
  • bool() → asBoolean()
  • toLong() → asLong()
  • toDouble() → asDouble()
  • string() → asString()
  • elements() → asList()
  • members() → asMap()
  • element(int) → get(int)
  • getOrAbsent(String) → tryGet(String)
  • valueOrNull() → tryValue()
• New method: asInt() on JsonValue and JsonNumber
• JsonArray: add get(int) override with bounds-checked error
• JsonObject: add get(String) and tryGet(String) overrides
• Remove equals()/hashCode() from all JsonValue subtypes

Exception Changes

• Rename JsonAssertionException → JsonValueException
• JsonParseException → make final, add non-negative validation for line/pos

Internal Implementation (jdk.sandbox.internal.util.json)

• JsonNumberImpl: add asInt(), new numInteger LazyConstant, use Math.powExact (Java 25+) or keep polyfill for Java 21
• JsonObjectImpl: rename members()→asMap(), add get()/tryGet() overrides, remove equals/hashCode
• JsonArrayImpl: rename elements()→asList(), add get(int) override, remove equals/hashCode
• JsonStringImpl: rename string()→asString(), remove equals/hashCode
• JsonBooleanImpl: rename bool()→asBoolean(), remove equals/hashCode
• JsonNullImpl: remove equals/hashCode
• Utils.java: return JsonValueException instead of JsonAssertionException, remove powExact polyfill if using Java 25+

Infrastructure

• Update API tracker URLs from java.util.json → jdk.incubator.json
• Update RefreshFromUpstream.java to use new path src/jdk.incubator.json/share/classes/jdk/incubator/json/impl/
• Update transform_upstream.py for new package names
• Update README.md "Current Status" section

Approach

1. Fetch upstream sources from json branch at src/jdk.incubator.json/share/classes/
2. Apply mechanical transformations (package renames, preview annotation removal, Java 21 compatibility)
3. Preserve local additions (StableValue.java, Utils.java polyfills)
4. Run full test suite
5. Update documentation