Add store overrides DSL

[hkusu]

Summary

• Add an overrides DSL for non-state store configuration while keeping Store { ... } and Store(initialState) { ... } usage intact.
• Support middleware replacement and clearing through replaceMiddlewares(...) and clearMiddlewares() inside overrides { }.
• Simplify middleware setup by allowing middleware(...) to accept one or more middleware instances.
• Add JVM tests for overrides behavior and update the README examples to use overrides instead of extraSetup.

Verification

• ./gradlew :tart-core:jvmTest