[v1.9.21] B. Build & test pass on Kotlin 1.9.21

[HarryJhin]

On the 1.9.21-release branch, get a full green build for Kotlin 1.9.21.

Acceptance criteria

• ./gradlew clean build -PkotlinVersion=1.9.21 succeeds
• Runtime tests (38) pass
• K1 box tests (10) pass
• Sample composite build (cd sample && ../gradlew run) prints expected output
• Any compiler-API differences vs. v1 HEAD are fixed only on this branch

Notes

• Record any API shape differences encountered in the PR description