Settlement policy: settle vs withdraw semantics

[greatest0fallt1me]

Description

Clarify product policy: does settle_stream move tokens immediately or only internal accounting until withdraw?

Requirements and context

• Single source of truth for UX and backend.
• Update rustdoc accordingly.

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 docs/settle-vs-withdraw-policy

Implementation targets

• docs/settlement-policy.md + rustdoc.

Tests and validation

• Review with backend.

• 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

docs(contracts): settlement vs withdrawal policy