Add `# Why largest-remainder?` section to `calc.rs` module doc

[Kingsman-99]

Description

contracts/split/src/calc.rs has a terse //! comment but does not explain why the largest-remainder method is used over simple floor division. Adding a //! # Why largest-remainder? section explaining rounding and the sum == total invariant would make the module self-sufficient for new contributors.

Acceptance Criteria

• Expand the //! module doc in contracts/split/src/calc.rs with a # Why largest-remainder? section
• Explain: floor division leaves up to n-1 stroops undistributed; LR assigns them to recipients with the largest fractional remainders
• Include a one-line numeric example (e.g. distributing 10 among 3 equal shares gives [4, 3, 3])
• cargo doc builds without warnings
• cargo test passes

Context

• Target file: contracts/split/src/calc.rs