Require JDK 26 bytecode in Kotlin modules

[gregjotau]

Summary

• compile the Thim compiler, example, and benchmark Kotlin modules to JDK 26 bytecode
• remove JVM target validation bypasses that hid Kotlin and Java target mismatches
• make the published compiler artifact match the documented JDK 26 minimum

Verification

• ./gradlew clean build (50 tasks)
• compiler JAR class-file major version: 70 (JDK 26)
• example tests and committed benchmark ceilings pass

Not merged; ready for manual review.