refactor: migrate Xtend to Java - com.avaloq.tools.ddk.check.core

[joaodinissf]

Summary

Migrates the last 8 .xtend files of com.avaloq.tools.ddk.check.core — the production Check framework — to Java 21, a two-step rename→translate structure plus a final commit dropping the module's Xtend build infrastructure:

File	Nature
CheckGeneratorConfig	config bean
CheckGeneratorNaming	naming helpers
CheckTypeComputer	type computation
CheckScopeProvider	scoping
CheckFormatter	formatting2 (47-branch dispatcher preserved verbatim)
CheckGeneratorExtensions	generator helpers (feeds emitted text)
CheckGenerator	HTML docs / IssueCodes / StandaloneSetup / ServiceRegistry emitter
CheckJvmModelInferrer	JVM model inference for check validators

Reviewing tip: the PR is structured as rename → translate: commit 1 is a pure git mv of the .xtend sources to .java (100% similarity, so git log --follow/git blame permanently traverse the migration), commit 2 rewrites them in place. Review commit 2 on the Commits tab to get a side-by-side Xtend-vs-Java diff per file; the aggregate Files-changed tab pairs only the near-transliterations. Commit 1 intentionally does not compile on its own.

The three byte-critical emitters keep the exact StringConcatenation call sequences of the Xtend compiler's own output (xtend-gen), including two-arg append(value, indent) and newLineIfNotEmpty() semantics — generated artifacts are byte-identical.

Infrastructure cleanup (module now fully off Xtend, matching the other migrated modules): xtend-gen/ removed from build.properties and .classpath, xtextBuilder/xtextNature removed from .project, xtend-gen/ directory deleted. No Require-Bundle change: the module declares no Xtend bundles; xtend2.lib/xbase.lib resolve transitively for the StringConcatenation-based generators, which remain by design.

Verification

• Per-file adversarial verification against a fresh xtend-gen ground-truth build: append-for-append comparison plus executable byte-equality harnesses (real StringConcatenation, batteries covering empty/single-/multi-line/quotes/%/unicode inputs) for all templates of the three emitters — all IDENTICAL (~150 harness cases).
• Dispatcher fidelity: instanceof-chain order diffed against xtend-gen — identical for all dispatchers (incl. the 47-branch CheckFormatter.format).
• Full local gate: clean verify checkstyle:check pmd:check spotbugs:check (PMD 7.26.0) → BUILD SUCCESS, 0 violations, 357 tests, 0 failures, 0 errors — identical to the pre-migration baseline on the same tree (one pre-existing SWTBot flake, testBulkApplyingQuickfix, appeared in a later re-run and is unrelated).
• Generation golden tests (IssueCodeToLabelMapGenerationTest, IssueCodeValueTest) green; no drift in any committed generated artifact.

Faithfulness notes

• Xtend += null-safety restored in CheckJvmModelInferrer: Xtend's += silently skips null; the plain Iterables.addAll translation let null JvmMembers reach the type resolver (IllegalArgumentException: element: null, reproduced by the CheckValidationTest severity-range stubs). The two affected call sites are wrapped in IterableExtensions.filterNull, like every sibling site.

Deliberate deviations (output-neutral)

1. Explicit StandardCharsets.UTF_8 in CheckGeneratorExtensions.getContents instead of the platform-default charset. Identical behavior on Java 18+ (JEP 400); the sole caller reads the service-registry file, which contains only ASCII fully-qualified class names, so emitted output is provably unchanged under any default charset.
2. IllegalStateException wrapping of checked exceptions there instead of Xtend's sneaky-throw. Differs only in the thrown type on I/O failure (the build fails either way); no effect on generated output.
3. .formatted() replaces the String.format the Xtend compiler emitted in splitCamelCase (repo convention; behavior-identical).
4. Completed Javadoc (@param/@return) on CheckGeneratorNaming/CheckGenerator methods whose originals carried plain block comments — required by the checkstyle JavadocMethod gate once the comments became Javadoc.

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