[HARD][SOROBAN ACCOUNTING] Use deterministic fair rounding when allocating pool units

[Obiajulu-gif]

Summary

Non-final investments receive floor(amount * total_units / target_amount), while the final funder receives every accumulated remainder. This makes allocation dependent on transaction order and can transfer rounding value to the last participant.

Repository evidence

• contracts/chainmove-pool/src/lib.rs floors proportional units in allocate_units.
• When new_total == target_amount, the function assigns total_units - funded_units to the final investment.

Scope

• Specify a deterministic rounding/remainder policy that preserves total units without favoring transaction order.
• Persist any remainder state required by that policy.
• Expose allocation evidence in events or receipts.

Acceptance criteria

• Equal economic inputs produce allocations within the documented rounding bound regardless of ordering.
• Allocated units never exceed the cap and sum exactly at full funding.
• No positive valid investment silently receives zero units without a documented minimum.

Tests

• Permutation/property tests across many investment orders and awkward ratios.
• Boundary tests near zero, full funding, and integer limits.

Non-goals

• Changing target amounts, asset custody, or investor eligibility.

[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 #170)
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 #209 was approved and merged by @Obiajulu-gif.

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