feat(json): add Kotlin JSON support

[chaokunyang]

Why?

Kotlin/JVM models carry constructor defaults, nullability, unsigned and value-class semantics,
sealed hierarchies, and complete generic type information that Java Class tokens cannot express.
This PR adds an optional Kotlin JSON module while keeping the existing Java JSON APIs and Fory
binary protocols unchanged.

What does this PR do?

• Adds fory-json-kotlin with ForyJsonKotlin.builder() and reusable jsonTypeRef<T>() tokens for
  Kotlin object models, defaults, nullability, generics, value classes, unsigned types, sealed
  hierarchies, Mixin overlays, custom codecs, and supported Kotlin/JVM built-ins.
• Extends the shared JSON resolver, object model, and generated capability paths so language-owned
  metadata can retain direct generated readers and writers, exact semantic types, nullability, and
  primitive/no-box value-class carriers.
• Adds fory-json-kotlin-ksp as a retention-only processor that emits exact R8/ProGuard rules for
  @JsonType and Kotlin-owned @JsonMixin declarations; it does not generate codecs or application
  classes.
• Adds a small shared JVM/Android/GraalVM corpus covering defaults, value classes and unsigned
  values, sealed types, Mixins, custom codecs, and generics, plus JPMS and release wiring.
• Adds Kotlin JSON documentation and a standalone Gradle/JMH benchmark project for Fory,
  kotlinx.serialization, Moshi, and Jackson Kotlin. Benchmark timing is not part of CI.

Related issues

N/A.

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence or equivalent persisted links of the final clean AI review results from both fresh reviewers described in AI_POLICY.md, the Fory-guided reviewer and the independent general reviewer, on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

Yes. It adds the optional Kotlin/JVM JSON runtime, Kotlin type-token and builder APIs, Android
retention-rule processor, documentation, and platform setup described above.

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark