fix!: align APT and KSP metamodel generation and make the Java processor incremental-safe

[zantvoort]

Aligns the Java (APT) and Kotlin (KSP) metamodel processors on one contract and makes the Java processor safe for Gradle incremental annotation processing.

Java processor quality

• META-INF/gradle/incremental.annotation.processors registers MetamodelProcessor and TypeIndexProcessor as aggregating, so attaching them no longer disables incremental compilation for the whole source set. Isolating is not an option: the instantiator services file and the type index aggregate cross-record state. The processors keep @SupportedAnnotationTypes("*") and root-element iteration deliberately — discovery is interface-based, and a record may implement Data/Projection with no annotations at all, so claiming specific annotation types would make javac skip the processor for such files in every build system. KSP discovers the same way through getAllFiles().
• A generation failure now reports an ERROR anchored to the failing record element, includes the stack trace, and rethrows to abort — the KSP semantics. The inner catch-alls are narrowed to IOException, so a codegen defect propagates with a source location instead of printing a bare Error: java.lang.NullPointerException and continuing.

APT/KSP divergence

• APT emits the <Type>NullableMetamodel chain variant for every record and sealed interface and selects child metamodel fields by derived nullability (explicit annotations, null-marked default), so Owner_.address has the same static type in Java and Kotlin. instance() stays on the base Data variant, validation diagnostics print once per record (gated to the base pass, in both processors), and the GraalVM feature already registers both class names, so native images need no change.
• KSP sources components from the primary constructor: a body-declared property (val display: String get() = ...) or an inherited one has no column and no longer receives a metamodel field pointing at a nonexistent column. Sealed interfaces contribute abstract properties only, matching APT's abstract-getter filter, and hasNullableLeaf walks the same component set.
• KSP escapes keyword-named properties (`object`, `fun`, …) at every identifier emission site; path and field string literals keep the raw name, so column derivation is unchanged. Java needs no counterpart: javac rejects keyword-named record components at declaration.

Verification

• New tests in both MetamodelProcessorTest suites: nullable child selection, nullable-variant emission and descriptor registration (Java); constructor-only components and keyword-named property compilation (KSP).
• Full reactor clean install green on JDK 21.

Fixes #411

[codecov]

Codecov Report

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