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

[joaodinissf]

Closes #1339.

Note

Stacked on #1396. The first commit (ci: fast static analysis…) belongs to #1396 and will drop out once that merges; review only the second commit here (chore: enable CPD…). I'll rebase to a clean single commit after #1396 lands.

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 — and the lint job's CPD gate (from ci: fast static analysis with early-fail lint + inline SARIF (PMD/Checkstyle/SpotBugs) #1396) becomes meaningful.
• At 100 the reactor surfaces 9 pre-existing duplications (5 file-groups). Each was assessed for whether it is genuinely extractable logic or incidental/deliberate similarity:
  • 2 refactored away (C, E) — genuine duplicated logic; behavior-neutral extraction.
  • 3 baselined (A, B, D) with // CPD-OFF markers carrying a per-site reason — incidental token overlap, or deliberately-explicit code where sharing would hurt readability.
• Verified: check.ui and xtext compile, and pmd:cpd-check is clean across the full reactor at threshold 100 (0 duplications).

The 9 duplications — disposition

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.

Notes

• CPD has no SARIF renderer, so it gates (the job goes red on a duplication) but doesn't annotate inline; a cpd.xml→SARIF transform for inline CPD is a possible follow-up.
• Branch-protection / required checks: unchanged by this PR (the lint/spotbugs rename is ci: fast static analysis with early-fail lint + inline SARIF (PMD/Checkstyle/SpotBugs) #1396's).

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