feat(kotlin): add schema idl support to kotlin

[chaokunyang]

Why?

Kotlin did not have Schema IDL compiler output or matching generated xlang registration support, so Kotlin users could not generate Fory IDL models alongside the other supported languages.

What does this PR do?

• Adds Kotlin as a Schema IDL compiler target, including --kotlin_out, kotlin_package, generator registration, and generated Kotlin model/registration output.
• Implements Kotlin IDL generation for structs, enums, unions, nested types, package handling, type IDs, unsigned types, half/bfloat16 arrays, duration fields, references, and cycle-owned classes.
• Extends Kotlin KSP/runtime support for generated xlang serializers, union serializers, runtime type aliases, generated serializer lookup, and Kotlin Duration xlang encoding.
• Adds Kotlin IDL unit and integration coverage, including a Kotlin peer module and Java round-trip hooks for the IDL test suite.
• Updates compiler, Kotlin, generated-code, schema IDL, and xlang type-mapping documentation for Kotlin support.

Related issues

#1017
#3679

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 of the final clean AI review results from both fresh reviewers on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

Yes. It adds Kotlin Schema IDL compiler output and related Kotlin generated-serializer registration APIs. It does not change the existing binary protocol format.

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

Benchmark

N/A. This PR adds Kotlin Schema IDL generation and coverage, not a benchmarked runtime optimization.