[Task] No maven-enforcer-plugin anywhere

## Description
No `requireMavenVersion`, `requireJavaVersion`, `dependencyConvergence`, `requireUpperBoundDeps`, or `bannedDependencies` (repo-wide grep: 0 matches).

## Location
```
repo-wide grep (0 matches)
```

## Impact
No build-time guardrail against version/convergence drift. This is why the version-drift and compiler-source findings went undetected.

## Suggested fix
Add enforcer to root `` enforcing Maven 3.6.3+, JDK 17, `dependencyConvergence`, `requireUpperBoundDeps`.

## Related existing issue(s)
None — #6372 is about dependency policy/process, not the missing enforcer mechanism.

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start with the repository root build configuration and review the existing section; the issue also links docs/issue-candidates-2026-08-02.md for audit context. Add the Maven Enforcer rules listed in the description, then run the project’s Maven build to verify the guardrails work without introducing dependency or version failures.

Written by the indexing model from the issue text.