classify_url does not recognize the final data.quantecon.org host — the audit breaks at the Phase 4 cutover

[mmcky]

Flagged during the 2026-08-10 step-3 gate work and noted as "untracked as of today" — still untracked until this issue. Filing it now because it becomes load-bearing at the Phase 4 cutover (#15).

classify_url in scripts/build_audit.py recognizes only the six GitHub-host URL forms. A read from the final hosting — data.quantecon.org/lectures/<file> or its interim Pages equivalent quantecon.github.io/data-lectures/lectures/<file> — classifies as external-web, so migrated comes out false. Two consequences:

• migration.yml's final status is a state the audit must currently report as broken: the moment any consumer flips interim → final, the strict audit sees an external-web read where it expects a data-lectures one.
• The host and resolvability assertions from Audit: assert on the media host, and on data-lectures URL resolvability #55 stop applying at exactly the cutover they were built to protect, because the URLs leave the recognized set.

Fix shape: teach classify_url the final host(s) as a first-class data-lectures classification (both data.quantecon.org and the quantecon.github.io/data-lectures form), and add a test that a final-status record with a final-host read audits green while an interim-host read under final status warns. Should land before the first consumer flips to a final URL, i.e. it gates the #15 cutover, not this week's Track C work.

See #15 (cutover runbook) and #55 (the assertions this preserves).