Rename ViewStore render and handle APIs

[hkusu]

Summary

• rename the ViewStore composable helpers from render and handle to stateContent and eventEffect
• keep render and handle as deprecated forwarding APIs to preserve source compatibility during migration
• update the README, internal notes, and JVM tests to use the new names

Why

• make the state rendering and event collection roles more explicit in Compose usage
• avoid the ambiguity of shorter names around the existing viewStore.state property

Impact

• existing call sites continue to compile through the deprecated wrappers
• new documentation and tests point users to the replacement APIs

Verification

• ./gradlew :koma-compose:jvmTest