Make the whole case library auditable: backfill routing cards for the 19 grandfathered cases

[openleek]

Why this matters

Every research case is supposed to carry a machine-checkable routing.json — the record of how the task was classified and scoped before any analysis. New cases are required to have one. But 19 older cases are grandfathered out via cases/legacy-routing-exempt.txt, so today the case library is split: new work is auditable, old work isn't. That's a blind spot — you can't ask "show me every actionability case that skipped the decision-pressure gate" and trust the answer while a third of the library is exempt.

This issue burns that exemption list down to zero, so the whole library answers routing questions consistently.

What's involved

• Backfill cases/<id>/routing.json for the 19 exempt cases, removing each from cases/legacy-routing-exempt.txt as it's done.
• Order: any canonical/golden or actively-referenced cases first (highest value), oldest one-off backtests last.
• Each backfill is validated instantly by just check-case <id> against schemas/routing.schema.json.
• When the list reaches zero, delete the file and the validate_case_routing exempt branch in validate_repo.py.

Done when

• cases/legacy-routing-exempt.txt is empty (then removed), and every case with a memo/evidence-log has a schema-valid routing.json.
• just check stays green throughout (it will, since each case is validated as added).

Context

The exempt list was a deliberate grandfather snapshot (PR #51) to avoid turning the schema rollout into a 21-case migration. It doubles as a visible debt counter — this issue is the burndown. See architecture/harness-productization-notes.md (WS1.5 / WS2).