docs: explain money representation

[Bhargi777]

Closes #155

Summary

• Document why on-chain stream IDs use BigInt and API strings.
• Explain decimal-string token amounts, base units, and explicit BigInt serialization.

Validation

• git diff --check passed.
• npx prettier --check docs/MONEY.md passed.
• npm run lint passed.
• npm run typecheck is blocked by the existing origin/main missing @fastify/compress dependency referenced by src/Plugins/compression.ts.