Stream transfer: change recipient with explicit authorization

[greatest0fallt1me]

Description

Allow recipient change only with auth from payer and new recipient (and old recipient if required).

Requirements and context

• Prevent hijack.
• Events.

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/transfer-stream-recipient

Implementation targets

• lib.rs — propose_recipient / accept or single-tx pattern.
• Tests for each auth combo.

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): recipient change with explicit authorization

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