chore: enable CPD detection (pmd.cpd.min → 100), de-dup and baseline

[joaodinissf]

Closes #1339.

Problem

ddk-parent/pom.xml set pmd.cpd.min=100000, which effectively disables PMD's copy/paste detector — pmd:cpd-check passes on virtually any duplication (#1339).

What this does

• Lowers pmd.cpd.min to 100 (PMD's default), so CPD actually detects duplication going forward.
• At 100 the reactor surfaces pre-existing duplications in 8 file-groups. Each was assessed for whether it is genuinely extractable logic or incidental/deliberate similarity:
  • 2 refactored (C, E) — genuine duplicated logic; behavior-neutral extraction.
  • 6 baselined (A, B, D, F, G, H) with // CPD-OFF markers carrying a per-site reason.
• Verified: full local gate (clean verify checkstyle:check pmd:pmd pmd:cpd pmd:check pmd:cpd-check spotbugs:check) → BUILD SUCCESS, pmd:cpd-check clean across the full reactor at threshold 100.

Dispositions

Group	File(s)	Kind	Disposition	Rationale
A	AbstractValidationTest (test core)	test scaffolding	baseline	error-message assembly + assert helpers in a test base; per-test clarity beats a shared extraction that would obscure intent.
B	ParameterListMatcherTest (typesystem test)	test cases	baseline	sibling testForceMatchByPosition* cases share an arrange-act-assert skeleton, kept explicit for readability.
C	Check{Validator,Quickfix}ExtensionHelper (prod UI)	helper pair	refactored	identical isExtensionUpdateRequired guard chain lifted to AbstractCheckExtensionHelper, keyed on the existing getExtensionPointId() plus a new protected getTargetClassName() hook. The documentation-helper subtree (no target class) declares that hook unsupported once in its shared parent.
D	DispatchingCheckImpl (prod runtime)	incidental	baseline	the matched spans are incidental token overlap (DI field declarations + the trace/try/run/catch/finally idiom), not an extractable unit.
E	Abstract{,Streaming}FingerprintComputer (prod)	helper pair	refactored	the identical private featureIterable helper extracted to a shared package-private FingerprintFeatures utility.
F	FormatJvmModelInferrer (prod, migrated)	dispatch/emission	baseline (file-scoped)	9 self-duplications across the per-locator emission blocks of freshly-migrated Xtend dispatch code, kept faithful to the migration; de-duplication belongs to the migration readability follow-ups, not this PR.
G	FormatFragment2 (prod generator, migrated)	stub emission	baseline	the parallel Xtend- and Java-formatter stub emission runs are deliberately explicit twins.
H	CheckCfg{ContentAssist,ConfiguredParameterValidations}Test	test scaffolding	baseline	sibling test classes share @Override setup scaffolding, kept explicit per class.

Notes

• The lifted isExtensionUpdateRequired overrides keep their public visibility: the hook is deliberately public so tests can exercise it directly on injected helpers; fix: make isExtensionUpdateRequired uniformly public (widen over narrow) #1460 makes the whole hierarchy uniformly public.
• CPD has no SARIF renderer, so it gates (the build goes red on a duplication) but doesn't annotate inline; a cpd.xml→SARIF transform is a possible follow-up.
• On the current workflow CPD gates via maven-verify's pmd:cpd-check; once ci: fast static analysis with early-fail lint + inline SARIF (PMD/Checkstyle/SpotBugs) #1396's lint lane lands, its CPD gate becomes meaningful too (its "threshold currently very high" note can then be dropped).

Merge chain: #1399 (merged) → #1396 → #1400 → #1456 → #1457 → #1397 (this PR)

🤖 Generated with Claude Code

[joaodinissf]

I will coalesce these in a follow-up PR.