[#544] Make fee accounting and rounding invariant-safe

[arisu6804]

Summary\n\nThis PR centralizes fee arithmetic and makes every supported fee lifecycle use a conservation-safe split.\n\n- adds checked quotient/remainder arithmetic that avoids intermediate multiplication overflow;\n- makes rate and amount validation explicit and fail closed;\n- returns gross, fee, net, and rounding remainder together;\n- supports optional persisted remainder carry for repeated dust-sized operations;\n- wires the policy into commitment creation and early-exit accounting;\n- wires the same policy into marketplace listings, offers, and auction settlement;\n- rejects invalid marketplace configuration instead of silently clamping it;\n- adds boundary, large-value, carry, ledger, and conservation tests;\n- documents rounding, lifecycle ordering, events, reconciliation, and upgrade rules.\n\nThe change set contains 521 added lines of implementation, tests, and accounting documentation.\n\n## Validation\n\n- \n- (blocked by the repository's existing Rust/toolchain incompatibility in dependency , before project code compilation)\n\nCloses #544