Use proper diff output on fmt --check

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[augmentcode]

🤖 Augment PR Summary

Summary: This PR enhances jsonschema fmt --check failure reporting with content diffs.

Changes:

• Links the CLI target against Sourcemeta Core's diff library.
• Computes a line-based Myers diff between the current and formatted document.
• Prints unified diff output to stderr for non-JSON check failures.
• Replaces JSON error-path strings with objects containing a path and structured diff operations.
• Serializes equal, deleted, and inserted line groups for JSON output.
• Shares reporting logic between stdin and file-based formatting checks.
• Updates single-file, multi-file, test-document, indentation, and stdin fixtures.

Technical notes: Expected formatting remains generated by the existing formatter; this change only alters failed-check diagnostics and the failure JSON shape.

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[augmentcode]

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[cubic-dev-ai]

All reported issues were addressed across 11 files

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