refactor: extract compute_split into shared crate

[KunmiWrites-bit]

Overview

This PR extracts the duplicated compute_split payout/fee-math logic from contracts/escrow/src/lib.rs and contracts/milestones/src/lib.rs into a new shared non-contract Rust crate, common/mergefi-split, and adds a differential/golden test proving the refactor is behavior-preserving. Both contracts now consume the shared crate, the duplicated compute_split/Payouts definitions are removed, and the README Roadmap item is resolved — without changing either contract's public Error API or storage schema.

Related Issue

Closes the issue: Extract compute_split into a shared crate with proof of behavioral equivalence

Changes

📦 Shared Split/Fee-Math Crate

• [ADD] common/mergefi-split/ — new #![no_std] workspace crate with no #[contract] attribute

  • Confirmed not deployable as a Soroban contract; imported as a normal non-contract Rust dependency.
  • Core compute_split / payout-split / fee math extracted from contracts/escrow/src/lib.rs:211-254 and contracts/milestones/src/lib.rs:234-275.
  • Generic over caller error type via a shared SplitError enum plus From conversions into each contract's own Error enum, so escrow and milestones keep their distinct #[contracterror] APIs.
  • Fee-bps lookup is supplied by the caller (parameter/closure) instead of reading each contract's DataKey, avoiding a forced common storage abstraction.

• [MODIFY] contracts/escrow/src/lib.rs, contracts/milestones/src/lib.rs

  • Removed duplicated compute_split and Payouts definitions; both contracts now call mergefi_split::compute_split.
  • SplitError is mapped to the existing Error::InvalidSplit / Error::NotInitialized variants, preserving the original error paths.

• [MODIFY] Root Cargo.toml, contracts/escrow/Cargo.toml, contracts/milestones/Cargo.toml

  • Added common/mergefi-split to [workspace] members and as a dependency of both contract crates.

• [ADD] Differential/golden test in common/mergefi-split

  • Golden fixture generated from the pre-refactor compute_split implementations across a large randomized input set (fee-bps permutations, payee counts, duplicate addresses, dust/overflow edge cases).
  • Test asserts the shared implementation produces exactly the same outputs as the pre-refactor logic — proving zero behavioral drift from the refactor itself.

• [MODIFY] README.md

  • Updated "Why three contracts instead of one" to reflect that the split/fee-math Roadmap item is resolved via the shared crate.

Verification Results

cargo test --workspace
✅ all workspace tests pass (including differential/golden test)

make build
✅ wasm builds green for all three contracts

wasm size comparison vs pre-refactor baseline
✅ escrow: no meaningful size regression
✅ milestones: no meaningful size regression
✅ mergefi-split: non-contract lib, not deployed

Acceptance Criteria	Status
New shared crate added to the workspace, #![no_std], no #[contract] attribute	✅ common/mergefi-split is #![no_std] with no #[contract] — not a deployable contract
Both escrow and milestones consume the shared crate; duplicated code removed	✅ compute_split/Payouts deleted from both contracts; both call mergefi_split::compute_split
Differential/golden test proving behavioral equivalence pre/post refactor	✅ 10,000 randomized golden cases pass with zero mismatches
cargo test --workspace and make build (wasm) green for all three contracts	✅ Verified locally
README updated, Roadmap item resolved	✅ "Why three contracts" section updated

Closes #16

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