refactor: modernize build system, redesign KSP processor, and automate Maven Central release

[y9vad9]

Why

The project had broken dependency resolution from old build-logic leaking AGP into KSP, several codegen crashes in common edge cases, and no working pipeline to publish to Maven Central.

Notable Changes

• Generated Immutable* classes from interfaces are now data classes.
• @BuilderImpl and @DSLBuilderImpl now work when either @ImmutableImpl or @MutableImpl is present (previously failed if both were not declared together).
• Nullable properties in builders now default to null instead of crashing at runtime on uninitialized Delegates.notNull().
• Invalid processor targets (concrete classes, missing realization annotations, blank names) now report compiler errors with source locations instead of crashing the compiler daemon with an unhandled exception.
• Codegen now passes originating KSFile dependencies to enable KSP incremental compilation.
• Publication is migrated from the deprecated custom deploy script to Maven Central using vanniktech-maven-publish via GitHub releases.

Testing

• Added unit tests covering all codegen targets, abstract classes, and builders (sample.ImplierTest).
• Added compiler diagnostic tests via Kotlin Compile Testing (sample.CompilerDiagnosticsTest) to verify compilation failures on invalid targets.
• ./gradlew check passes cleanly.