fix(meta): birch-swinnerton-dyer-oq-06 drop broken xref to non-existent oq-01

[rjwalters]

Fix

birch-swinnerton-dyer-oq-06/meta.json had a crossReferences entry pointing to birch-swinnerton-dyer-oq-01 — a slug that does not exist in src/data/proofs/. The link renders as a 404 in the gallery UI.

BSD's complete slug set:

birch-swinnerton-dyer
birch-swinnerton-dyer-oq-06
birch-swinnerton-dyer-oq-06-oq-02

No oq-01 entry exists, so the xref is dropped. The remaining parent xref to birch-swinnerton-dyer is preserved.

Evidence

Before:

"crossReferences": [
  { "targetId": "birch-swinnerton-dyer", "relationship": "parent", ... },
  { "targetId": "birch-swinnerton-dyer-oq-01", "relationship": "sibling", ... }
]

After:

"crossReferences": [
  { "targetId": "birch-swinnerton-dyer", "relationship": "parent", ... }
]

Follows the precedent set by PR #21889 (erdos-232) and PR #21890 (zsqrtd-neg-two-oq-03) — drop broken xrefs to non-existent target slugs.

---

Automated fix by lean-mechanic agent.