Clarify deterministic output terminology

[Will-thom]

Summary

• add a deterministic output glossary for text and JSON output
• explain stable text ordering, stable JSON structure, and compatibility expectations
• link the terminology from README, project context, and JSON contract docs

Validation

• git diff --check
• .github/scripts/check-public-metadata.sh
• go test ./...

Closes #61

[github-actions]

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