Document token list contributions

[JOY (JOY)]

Summary

• add a DOS Chain token-entry JSON template and dedicated PR checklist
• document on-chain metadata, icon, versioning, and evidence requirements
• reject duplicate and zero token addresses in CI
• harden SVG and PNG validation with regression coverage

Validation

• 14 Python regression tests passed
• current Mainnet token assets passed
• ESLint passed
• Uniswap Token Lists schema passed
• actionlint passed
• git diff --check passed
• independent reviewer approved after all Important findings were fixed