Add CenteredHeadline CV v2 preset

[DemchaAV]

Summary

Adds the CenteredHeadline CV preset to the cv/v2 layered architecture with isolated theme tokens, reusable Subheadline and SectionHeader.flatSpacedCaps widget support, visual baselines, docs, and smoke coverage. Also corrects the shared v2 education sample dates to periods, updating existing v2 visual baselines intentionally.

Type of change

• Bug fix (no public API change)
• New feature / public API addition
• Documentation only
• CI / build / tooling
• Refactor (no behavioural change)

Checklist

• PR targets develop (the integration branch); not main.
• Branch name follows <type>/<short-description>: feature/cv-v2-centered-headline.
• ./mvnw -B -ntp clean verify passes locally.
• Java 17 compatible — no List.getFirst() / getLast(), no Thread.threadId(), no switch type / deconstruction patterns, no case null, default ->.
• Public API change: CHANGELOG.md entry added under ## v1.6.5 — Planned.
• README touched: DocumentationCoverageTest passes as part of clean verify.
• Examples touched: ./mvnw.cmd -f examples\pom.xml test passes and regenerates the example gallery smoke outputs.

Validation

• ./mvnw.cmd -B -ntp clean verify -> 894 tests, 0 failures/errors/skips, build success.
• ./mvnw.cmd -f examples\pom.xml test -> 1 test, 0 failures/errors/skips, build success.
• git diff --check -> no whitespace errors; LF->CRLF warnings only.
• CvV2VisualParityTest passed after approving intentional baselines.

Linked issue

N/A