fix: distribute split dust by largest remainder

[bingmokaka]

Summary

• replace the last-recipient dust assignment in escrow and milestone compute_split with largest-remainder allocation
• preserve the no-stranded-dust invariant while removing the previous position-dependent windfall
• document the rounding behavior, dust upper bound, and stable tie-breaking in the README
• add regression tests for a split where the old algorithm would have paid the final recipient extra dust

Testing

• rustup run stable-x86_64-pc-windows-gnu cargo check -p mergefi-escrow -p mergefi-milestones --target wasm32v1-none

I also attempted host tests with both MSVC and GNU Rust toolchains on this Windows machine. They did not complete because the local environment lacks MSVC SDK libs for the MSVC linker and then hit a Windows cdylib export ordinal linker issue under GNU. The contract code type-checks for the wasm target, and the PR includes focused unit tests for the changed behavior.

Fixes #6

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