test: fuzz core fee arithmetic invariants

[dangelo352]

Summary

• Make the commitment_core fuzz fee helper use quotient/remainder math so valid max-sized inputs do not overflow before division
• Add deterministic seed-grid coverage for fee <= amount and net_amount + fee == amount across boundary bps values
• Cover invalid domains, zero-fee, full-fee, and i128::MAX-adjacent amount observations
• Document the fee arithmetic invariants in SECURITY_AUDIT_PREP.md

Closes #486.

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-fuzz-target cargo test -p commitment_core fuzz -- --nocapture
• git diff --check

Note: Rust formatting was applied only to the touched fuzz files because repo-wide cargo fmt --check currently reports unrelated pre-existing formatting drift.