fix: use instance Velocity engine for appended resources

[efegokdemir]

Fixes #269

Summary

Use the per-execution VelocityEngine when rendering appended .vm resources instead of the process-wide static Velocity singleton. This keeps the appended-resource path consistent with normal bundle rendering and avoids shared global state in the thread-safe mojos.

Changes

• Replace static Velocity.init/Velocity.evaluate calls with the configured instance engine.
• Add coverage for filtering an appended .vm resource.

Testing

• mvn -Dspotless.check.skip=true -Dspotless.apply.skip=true test — passed, 12 tests.
• git diff --check — passed.
• Checkstyle and RAT — passed.

The repository Spotless check is blocked locally by the installed JDK and the bundled palantir-java-format (NoSuchFieldError in JCTree$JCCompilationUnit).