Add test for rounded-zero protocol fee

[Killerjunior]

Closes #794

Description
This PR adds a focused contract test covering the case where a non-zero protocol fee rate produces a rounded fee of zero for very small amounts.

What changed

• Added a single sentence of documentation in lib.rs
  • clarifies that when fee calculation truncates to zero, no transfer or fee_collected event occurs and the original amount is returned unchanged
• Added a new test in test.rs
  • test_no_fee_transfer_or_event_when_fee_rounds_to_zero
  • verifies a tiny stream amount with a non-zero fee rate does not transfer funds to treasury
  • ensures no fee_collected event is emitted

Why

• strengthens protocol fee behavior guarantees
• covers the edge case where fee rounding would otherwise imply an incorrect treasury transfer or event emission
• prevents regressions for minimal stream amounts under the current fee calculation

Testing

• contract test added in test.rs

Notes

• This PR is intentionally scoped to the rounded-zero fee edge case only.
• No frontend or unrelated backend files are included.