feat; [Test] Assert fee math prevents zero-loss rounding errors

[Happybello365]

Description
If fees calculate rounding errors, token fragments become permanently stranded representing lost capital value.

Explicit Expectations

Spin test paths confirming math where splits involve odd un-divisible token subsets.
Ensure the protocol absorbs loss fragments rather than sticking users.
Acceptance Criteria

Dust tokens settle predictably on the protocol balancing variables.
Contributor checklist:

Ensure the project builds successfully.
Add or update tests for all new/changed logic.
Make sure all tests pass.
Run the formatter / linter so code is properly formatted and consistent.
closes #349