fix: use checked creation fee arithmetic

[dangelo352]

Summary

• route commitment_core creation-fee calculation through the checked bps helper
• keep guard reset behavior on arithmetic overflow before failing as ArithmeticOverflow
• document the checked creation-fee source of truth

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-fee490-target cargo test -p commitment_core create_commitment_fee -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-fee490-target cargo test -p commitment_core test_create_commitment_fee_overflow_resets_reentrancy_guard -- --nocapture
• git diff --check

Closes #490

[dangelo352]

Closing as superseded by #519, which already implements the checked creation-fee path for #490 and is green with overflow/guard-reset coverage. I opened this duplicate while continuing the bounty queue; keeping #519 as the canonical PR.