IMG-016: No Output Validation

[murdore]

Summary

process() method doesn't validate generated data URI before returning.

Root Cause

Lines 22-38 don't validate output format, base64 content, or MIME type.

Fix

Add output validation checking data URI format, base64 non-empty, MIME type in allowed list.

Acceptance Criteria

• [ ] Validate data URI format
• [ ] Check base64 portion is non-empty
• [ ] Verify MIME type is in allowed list
• [ ] Add tests for output validation