[HARD][SOROBAN ACCOUNTING] Prevent partial-refund rounding from preserving excess units

[Obiajulu-gif]

Summary

Partial refunds floor the proportional units removed on every call. Repeating small refunds can recover principal while retaining more units than the remaining investment economically supports.

Repository evidence

• contracts/chainmove-pool/src/lib.rs computes refund_units = amount * position.units / position.invested with integer division.
• The floored result is subtracted independently on each partial refund, with no carried remainder or post-refund ratio invariant.

Scope

• Define an exact post-refund unit entitlement or carry deterministic rounding remainder.
• Enforce pool/position unit-to-principal invariants after every refund.
• Document minimum refundable granularity and terminal dust handling.

Acceptance criteria

• Splitting one refund into many calls cannot preserve extra units versus an equivalent single refund.
• A full refund always removes all remaining units.
• Pool funded units equal the sum of position units under tested transition sequences.

Tests

• Property tests comparing one-shot and partitioned refunds across awkward ratios.
• Dust, final refund, overflow, and interleaved funding/refund tests.

Non-goals

• Changing initial funding allocation policy.

[prissca]

i will love to work on this issue

[grantfox-oss]

🦊 GrantFox — @prissca has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #166)
2. Your PR will be reviewed by the Chainmove maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@prissca's PR #210 was approved and merged by @Obiajulu-gif.

🏆 @prissca: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,222 total points). Track your full progress on GrantFox.

👏 Great work, @prissca! Keep contributing to Chainmove.