Report processed annotation counts

[oiahoon]

Summary

• count emitted error and warning annotations
• report the final counts to STDERR after processing completes
• keep GitHub annotation output on STDOUT unchanged
• update the test harness to assert STDOUT and STDERR independently

Why

An empty STDOUT could mean either that a report contained no errors or warnings, or that processing produced no visible result. A final STDERR status makes successful processing explicit without changing the annotation stream consumed by GitHub Actions.

Closes #28.

Validation

• all 19 existing report and option scenarios pass
• mixed report keeps the same three STDOUT annotations
• mixed report writes Processed errors: 2; warnings: 1. to STDERR
• empty reports write zero error and warning counts
• malformed XML retains its existing STDERR diagnostics
• PHP syntax checks for cs2pr and tests/tests.php
• Composer manifest validation
• repository PHP CS Fixer dry-run
• Git diff check