Add denomination migration path when payment_token decimals change post-launch

[thlpkee20-wq]

Description

Some payment tokens migrate to new decimals. Provide a controlled migration path that re-scales stored amounts and updates metadata under issuer authorization.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: src/lib.rs
• Migration must be idempotent per (offering_id, from_decimals, to_decimals)

Suggested execution

• Fork the repo and create a branch
• git checkout -b feat/denom-migration
• Implement changes
  • Add migrate_denomination(offering_id, from_dec, to_dec)
  • Re-scale stored amounts with checked arithmetic
  • Emit denom_migrated event
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test --all
• Cover edge cases
  • Downscale that would round to zero for smallest balances
• Include test output and security notes

Example commit message

feat: add denomination migration path

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

[Gentleslyl2006]

Hello maintainer
I would love to Fix this Issue and will drop PR immediately