Use deterministic fair rounding for pool units

[prissca]

What changed

• allocate units from cumulative pool entitlement instead of flooring each contribution independently
• carry fractional value through the cumulative numerator and remove the final-funder remainder windfall
• reject below-granularity investments with an explicit contract error
• emit allocation evidence with awarded units and the post-allocation remainder numerator
• add awkward-ratio, ordering-bound, cap, and minimum-granularity tests

Rounding policy

At every transition the pool entitlement is floor(total invested × total units / target). Individual orders may differ by at most one indivisible unit, while a fully funded pool always sums exactly to the unit cap.

Validation

• Rust sources parse successfully with rustfmt
• contract tests could not link on this machine because the MSVC link.exe build tool is not installed

Closes #170

[prissca]

Thanks for the contribution! Merging this now as part of a maintainer PR sweep.

[Obiajulu-gif]

Hi! This PR currently has a merge conflict with the base branch and can't be merged as-is. Could you please rebase/merge main into your branch and resolve the conflicts? Thanks!

[Obiajulu-gif]

Hi! This PR currently has a merge conflict with the base branch and can't be merged as-is. Could you please rebase/merge main into your branch and resolve the conflicts? Thanks!

[Obiajulu-gif]

Thanks for this PR! It currently has merge conflicts against main and can't be merged as-is. Could you rebase/merge main into your branch and resolve the conflicts? I'll take another look once it's conflict-free.