Fixed-point rates for Stellar asset decimals

[greatest0fallt1me]

Description

Support fixed-point rate representation to align with 7-decimal Stellar assets and avoid float in clients.

Requirements and context

• Document scale factor in StreamInfo or separate field.
• Migration from raw i128 rate if breaking.

Suggested execution

• Fork the repository and create a branch (example below).
• Implement the changes in the StreamPay-Contracts Soroban crate (streampay-contracts).
• Keep the contract secure, tested, and documented; prefer small, reviewable diffs.

git checkout -b feature/fixed-point-rate-decimals

Implementation targets

• lib.rs + docs/decimals.md.
• Tests with realistic decimals.

Tests and validation

• Add or extend contract tests for this scope.

• Cover edge cases; paste summarized test output and security notes in the PR.

• Run cargo test in StreamPay-Contracts.

• Cover edge cases; capture test output and short security notes in the PR description.

Guidelines

• Target minimum 95% test coverage for touched contract code (or justify gaps explicitly).
• Clear documentation (rustdoc / project docs as specified).
• Timeframe: 96 hours from assignment.

Example commit message

feat(contracts): fixed-point rate for asset decimals