Add removed connection checkstyle rule

[sfc-gh-pvillard]

Summary

• add a default removedConnection checkstyle rule that reports connection UUIDs removed between flow versions
• preserve existing checkstyle callers while passing the previous snapshot to rules
• support nested/deleted process groups, component exclusions, and existing include/exclude controls
• document the rule and add focused unit and integration coverage

Test plan

• mvn -f flow-diff/pom.xml test (64 tests)
• git diff origin/main...HEAD --check
• changed Java files checked for package/import/brace issues, suppressions, conflict markers, and ticket references

[sfc-gh-dchaffelson]

LGTM — clean, backward-compatible feature with genuinely thorough test coverage (nested/deleted process groups, UUID-identity semantics incl. delete-and-redraw, null-previous first-version safety, and all three exclusion levels). The 3-arg getCheckstyleViolations overload keeps existing callers untouched, and the null-previous guard in RemovedConnectionRule is correct.

One non-blocking question: removedConnection is registered as a default rule, so it'll fire for all existing default-checkstyle consumers whenever a connection is legitimately removed. Assuming on-by-default (with exclude as the opt-out) is the intended UX rather than opt-in — just flagging in case you'd want it opt-in.