Implement diagnostics: --benchmark, --list_rules, --explain, and -X

[Matographo]

Summary

Add benchmark output, rule inspection, setting and rule explanations, and debug logging without corrupting render output.

Why

A performance-oriented CLI needs observability and introspection without sacrificing automation friendliness.

Acceptance Criteria

• Diagnostic output goes to stderr when appropriate
• Inspection commands are stable and documented
• Render output remains predictable

Dependencies

• Depends on Specify CLI behavior, inspection commands, and exit codes #2
• Depends on Implement deterministic rule precedence resolution #13
• Depends on Implement the settings namespace for -s / --settings #15
• Depends on Implement the core renderer for strings and single files #22