L2 gate: a regenerated gen/ file is read as a hand edit #6226

Refs #6063, #6198.

Problem

.github/workflows/l1-traceability.yml step "Check L2 GENERATION" fails any PR whose diff has M gen/.... It cannot tell a hand edit from a file regenerated out of a spec changed in the same PR. #6218 changed specs/tri/t27b/steward.t27, regenerated gen/c/tri/t27b/steward.c (byte-identical to t27c gen-c on the t27c lab's master build), and went red. The only way past it is a reviewer reading the check as noise, which trains everyone to ignore L2.

Proposal

For each modified gen/c/<path>.c, regenerate it from its spec with the PR's own t27c and compare bytes. Equal is a regeneration (pass); different is a hand edit (fail, as today). Spec lookup: the generated file's header, or a gen->spec map.

Acceptance

• A PR that changes a spec and its regenerated gen/ file passes L2.
• A PR that edits a gen/ byte by hand still fails (mutation control).
• No t27c in the job means "not checked", printed, never a pass.