Limit StoreBuilder helper visibility

[hkusu]

Summary

• mark StoreBuilder helper wrapper types as @PublishedApi internal
• mark StoreBuilder and StateHandlerConfig helper registries as @PublishedApi internal
• keep the current inline DSL behavior while removing these helpers from the normal public surface

Why

• narrow the accidentally exposed implementation details in StoreBuilder without changing behavior

Validation

• Reusing configuration cache.

Task :koma-core:kmpPartiallyResolvedDependenciesChecker
Task :koma-core:checkKotlinGradlePluginConfigurationErrors SKIPPED
Task :koma-core:transformCommonMainDependenciesMetadata UP-TO-DATE
Task :koma-core:compileCommonMainKotlinMetadata UP-TO-DATE

BUILD SUCCESSFUL in 387ms
3 actionable tasks: 1 executed, 2 up-to-date
Configuration cache entry reused.