test: cover never-executed fn in aprender-train/src/transformer/norm.rs #5531

Test-only ticket (<=200 lines). Source: coverage-nightly run 39080471983 (sha e6ab79f), ranked by never-executed function size; pmat query --coverage-gaps returned nothing on that JSON (CI absolute paths), so the ranking is by my jq over the same file, not pmat's impact score. crates/aprender-train/src/transformer/norm.rs has a function never executed in that run (~120+ lines of region span).

Do: run pmat query "<name>" --coverage --uncovered-only locally on this file, write tests for the uncovered function(s) (behaviour, not just no-panic), and per the coverage+contracts rule add or strengthen the matching falsification condition in the contract YAML (pv validate). No unwrap, no new Python.
Done when: the function is executed by a test in cargo test -p <crate> --lib and the contract line is in. Before starting: confirm origin/main still has the function uncovered and no branch carries this ticket number.