test: add JSON round-trip and snapshot tests

[rocketman-code]

Context

Parent: #160

JSON field naming inconsistency (#149) was missed because tests check individual fields exist but never validate that struct field names match the serialized JSON schema end-to-end. Report types have to_json() and to_terminal() methods but no tests verify the full output shape.

Expected Behavior

Tests that serialize reports to JSON, deserialize into serde_json::Value, and assert the complete field structure matches a spec. Optionally, snapshot tests (via insta) that catch any unintentional output changes.

Scope

• Add round-trip tests for every report type: TraceReport, ChainReport, CutReport, DiffReport, PackagesReport
• Each test: build a report from a fixture, call to_json(), parse the JSON, assert all field names and types
• Consider adding insta for snapshot testing of both JSON and terminal output
• Assert field name consistency across report types (e.g., weight fields always use _bytes suffix)
• Assert that to_terminal() and to_json() contain equivalent information