Validate key length when checking dependencies

Problem/Motivation

Identified by Claude Fable:

`checkDependencies($text, $key)` receives the key but only checks that Halite exists. A wrong-length key therefore isn't caught at profile-save validation; it surfaces later as a runtime `InvalidKey`. Adding a 32-byte length check here gives a clean validation error instead (length isn't secret, so a plain comparison is fine).