Output: include metadata object in JSON output

[unclesp1d3r]

Summary

JSON output is missing the metadata object specified in Core Flow 8 (JSON Output with Metadata).

Context

The spec defines JSON output should include:
```json
{
"filename": "sample.bin",
"matches": [...],
"metadata": {
"file_size": 8192,
"evaluation_time_ms": 2.3,
"rules_evaluated": 45,
"magic_file": "/usr/share/file/magic/Magdir/"
}
}
```

Current state: JSON output only includes filename and matches. The metadata fields exist in EvaluationResult and EvaluationMetadata but are not serialized to JSON output.

Acceptance Criteria

• JSON output includes metadata object with file_size, evaluation_time_ms, rules_evaluated
• magic_file path included when available (None for built-in rules)
• JSON Lines format for multiple files also includes metadata per file
• Existing JSON integration tests updated
• New tests for metadata field accuracy

Files to Modify

• src/output/json.rs - Include metadata in serialization
• src/output/mod.rs - Ensure metadata flows through to formatter

References

• Core Flows spec: Flow 8 (JSON Output with Metadata)
• Epic Epic: libmagic-rs Phase 1 MVP #24

[traycerai]

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