[#555] Enforce NFT royalty accounting and transfer invariants

[Jagadeeshftw]

Summary

• add seller-authorized royalty configuration with a 10% policy cap
• centralize fee, royalty, and seller payout calculation with checked arithmetic and exact value conservation
• apply the same accounting to primary sales, offers, and auction settlement, including cleanup and duplicate-settlement protection
• add real Soroban token settlement tests for rounding, zero/max royalty, authorization, rollback, and cancellation cleanup
• document the rounding and compatibility policy

Validation

• rustfmt --edition 2021 --check contracts/commitment_marketplace/src/lib.rs contracts/commitment_marketplace/src/tests.rs ✅
• git diff --check ✅
• cargo test -p commitment-marketplace ⚠️ blocked before project compilation by the existing ethnum v1.5.2 compile error (cannot transmute between types of different sizes) in the dependency cache

Refs #555