fix: enforce checks-effects-interactions ordering

[dangelo352]

Summary

• marks release, redirect, and cancel vaults terminal before invoking the external token transfer
• adds terminal-state regression coverage proving second release/redirect/cancel paths do not pay another balance delta
• documents the reentrancy threat model and links it from the USDC integration notes

Closes #234

Validation

• cargo fmt --check
• git diff --check
• cargo test

[1nonlypiece]

thanks for jumping on this. #234 hasn't been assigned to you though, and we need the PR author claimed on the issue before review. closing as invalid for now — grab #234 on the board and raise it again. https://discord.gg/xvNAvMJf