Flip the business_cycle snapshots to repointed once lecture-wasm reads them (P4 block C) #146

Part of #118, the tracker for the wasm-dynamic-snapshots project. This is block C, this repo's half of lecture-wasm's adoption of the four business_cycle snapshots. When the lecture-wasm PR for QuantEcon/lecture-wasm#70 (block B) merges, that repo's lectures/business_cycle.md starts reading gdp_growth_annual.csv, unemployment_rate_annual.csv, private_credit_to_gdp.csv and us_business_cycle_monthly.csv, and the records here have to say so on the same day. The strict audit fails on a landed record that has a reader, and on a repointed record that has none, so there is no green state between B and C (AGENTS.md, "Repoint every consumer of a dataset together").

Gate: B merged. Land this the same day, and not across a Monday. Between the two merges, every push to main, the Monday 05:17 UTC audit run and any PR that touches a manifest go red, and the Pages deploy is skipped. The PR can be opened before B merges. Its audit-dashboard check scans lecture-wasm's main, so it stays red until B merges and then needs a re-run.

What the PR changes

• The four manifests (lectures/<file>.yml): consumers becomes one entry, {repo: QuantEcon/lecture-wasm, file: lectures/business_cycle.md, on_refresh: <decided>}, replacing each "None yet" comment.
• lectures/gdp_growth_annual.csv.yml: also rewrite the three comments that state facts a refresh has since changed, which AGENTS.md forbids: source.version, the comment above retrieved, and the comment above integrity.upstream.status.
• migration.yml: the four records go from landed to repointed, each with repoints: [{repo: QuantEcon/lecture-wasm, pr: QuantEcon/lecture-wasm#<B>, date: <B's merge date>}]. Update the comment on the gdp_growth_annual.csv record too.
• scripts/audit_annotations.yml: delete lecture-wasm:business_cycle:wbgapi and lecture-wasm:business_cycle:pandas_datareader, which do nothing once B turns the API-teaching cells into non-executing blocks. Also update the note on intro's pandas_datareader entry, which says the snapshot pipeline is unadopted.
• CATALOG.md: regenerate it with python scripts/build_catalog.py, because the required consumed-files check fails on a stale catalog. The four files leave "awaiting repoint".
• PLAN.md: record the flip on P4's line in Phase 8, and tick that line if the canary proof has closed. Record on the consumer fan-out line in Phase 5 that a consumer which executes at read time needs no dispatch, as lecture-wasm dynamic snapshots: business_cycle reads the four data-lectures snapshots (P4) #118 sets out under "Not in scope". docs: close-out tidy — status, convention pointers and stale facts after the migration #145 also edits PLAN.md, so rebase on whichever lands first.
• If they are still there: delete the stale pending: P4 wave in migration.yml ("a maintained UNRATE twin"), which makes the migration page show "Wave P4" twice. Also rewrite the closing comment that says the final cutover is tracked at Phase 4 cutover: swap interim raw URLs → data.quantecon.org/lectures #15 and gated on DNS. D11 retired that cutover, and final now means qeld adoption.

Checks before merging

• Dry-run the strict audit in both directions, which is the P3 wave's acceptance test recorded in migration.yml. Shallow-clone the eight lecture repos into repos/, with lecture-wasm at B's branch until B merges and at main afterwards. Then run python scripts/build_audit.py all --strict --repos-dir repos -o site. Expect exit 1 with the four records at landed and exit 0 with them at repointed.
• In that run's site/, check three things: migration.html counts the four as migrated and shows one "Wave P4", and the migration legend on the overview page (index.html) has no negative count. The migration close-out corrects that legend while the four are still landed, so check that the correction holds after the flip.
• validate-datasets is green. The shared validate() reads every manifest this PR edits.

The decision it records

on_refresh for lecture-wasm is rebuild when no prose in B's final text quotes a number a refresh can move, and review when some does (manifest-schema.yml). Proposed: rebuild. Consumer fan-out is out of scope, so the value sets nothing off: lecture-wasm executes in the reader's browser and reads raw/main at run time.

Done when the PR has merged and the next audit-dashboard run on main is green and deploys.