Make command output blocks and exit codes consistent

[TomasVotruba]

Aligns how commands report results.

• duplicated-code: empty state uses the success() [OK] block (was green()); findings summary uses the error() block (was redBackground()), matching the other detection commands
• find-multi-classes: add a closing summary block before returning error, so it ends like the other detectors instead of trailing off; also fix "1 classes"/"1 files" plurals
• split-config-per-package: it used to finish silently - now prints a success summary with the number of per-package files written and the output dir; also returns ExitCode::SUCCESS instead of a literal 0
• privatize-constants: informational counts and empty states no longer use success()/warning() blocks that mismatched the exit code - neutral counts print plainly, empty states use a clear success message

No exit-code behavior changes here (spot-lazy-traits was intentionally left out, since it is being removed in #146).

ECS and PHPStan pass.