Hard cap on concurrent streams per payer

[greatest0fallt1me]

Description

Optional limit on active streams per payer to bound storage griefing.

Requirements and context

• Counter in storage; decrement on terminal states.
• Document behavior when cap reached.

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/per-payer-stream-cap

Implementation targets

• lib.rs.
• Tests at cap boundary.

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): configurable cap on streams per payer