fix: use checked creation fee math

[dangelo352]

Summary

• route commitment_core creation-fee calculation through fuzzing::checked_fee_from_bps
• preserve valid net amount behavior while mapping fee multiplication overflow to ArithmeticOverflow with guard cleanup
• extend the overflow regression so a later valid create succeeds after the failed overflow attempt
• update fee docs/cross-check to name the checked creation-fee source of truth

Closes #490.

Validation

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

Additional note: CARGO_TARGET_DIR=/tmp/commitlabs-fee-target cargo test -p commitment_core still has 3 unrelated pre-existing failures in emergency_tests::test_emergency_mode_toggle_emits_events, tests::test_create_commitment_event, and tests::test_create_commitment_updates_storage_layout. The focused overflow regression passed.