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

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 #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).

Start in scripts/build_audit.py at classify_url, then trace the existing audit tests and migration.yml status handling. Verify coverage for final-host reads and the final-status behavior described in the issue, including the warning for an interim-host read. Done means final-host records audit green at final status while interim-host records warn, preserving the assertions from #55 before the #15 cutover.