test: cover core settlement reentrancy guard

[dangelo352]

Summary

• wire the existing commitment_core security review tests into the test harness
• update two stale security review assertions to the current is_authorized API
• add regression coverage that settle and early_exit reject with an active reentrancy guard and clear the guard on validation rejections
• document the settlement-path reentrancy invariant

Closes #487.

Validation

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