fix(seo): B1 KILLED — Common Crawl backlinks measured, not assumed

The plan said Common Crawl was the free backlink source and the 70/100 cap
was therefore mandatory. Measured before building, and both premises die.

HEAD against data.commoncrawl.org, live:
  cc-main-2026-feb-mar-apr-domain-edges.txt.gz    17.3 GB   gzipped
  cc-main-2026-feb-mar-apr-domain-ranks.txt.gz     2.3 GB
  cc-main-2026-feb-mar-apr-domain-vertices.txt.gz  879 MB

Finding one domain's inbound links means scanning the edges file end to end,
per audit. That is not slow, it is non-viable — and abusive toward a
nonprofit serving the data free.

Worse, the reference implementation everyone points at
(claude-seo scripts/commoncrawl_graph.py:169) does this:

    max_compressed_bytes = 500 * 1024 * 1024   # 500 MiB safety cap
    if total_downloaded > max_compressed_bytes: break

500 MiB of 17.3 GB is **2.9% of the edges file**, which is sorted by source
ID — so it reads an arbitrary slice of source domains and reports whatever
backlinks happened to be in it, as a backlink profile, capped at "70/100
health". Nothing in the output says 3%. That is a random sample wearing a
measurement's clothes: the exact failure class this branch exists to remove,
and I was one step from copying it.

B2 dies with B1: nothing left to cap.

CONSEQUENCE, and it is the point: I1's narrowed Off-page axis — brand
mentions only, backlinks + authority declared unauditable in §14 — is the
FINAL state, not a placeholder waiting for data. Corrected my own I1 text,
which pointed at Common Crawl as the "nearest free source": that sends a
future reader into a 17 GB dead end. The §14 line now records what was
measured and why no number beats a fabricated one.

Also corrects the B3 note, whose follow-on ("so Common Crawl is the only free
source") was wrong for the same reason. The only free viable backlink source
is Bing's GetUrlLinks — first-party only, never a competitor, and blocked on
the client's Bing account. That raises W2's value; it does not unblock it.

Verified: full suite green, seo-data 144 pass / 0 fail.

.claude/tasks/TODO.md

@@ -13,8 +13,10 @@ NEXT: H1 (SSRF/injection guard) → C1 (sitemap crawl). Human merge gate: all
 - **B3 KILLED** — GSC Links API does not exist. Verified against the API
   reference: Search Console v1 exposes exactly Search Analytics, Sitemaps,
   Sites, URL Inspection. A subagent hallucinated it; I doubted it in the
-  plan and the doubt was right. Common Crawl is the ONLY free backlink
-  source → the 70/100 cap is mandatory, not optional.
+  plan and the doubt was right. (Its follow-on — "so Common Crawl is the
+  only free source, and the 70/100 cap is mandatory" — was ALSO wrong: see
+  B1/B2 KILLED below. Common Crawl is a 17 GB dead end, and Bing's
+  GetUrlLinks is the only viable free source, first-party only.)
 - **I1 was an over-correction** — "Off-page has ZERO data" was overstated
   (relayed from a subagent, unverified). Brand mentions ARE gathered
   (STEP 6). Narrowed the axis definition instead of N/A-ing it; weights
@@ -30,6 +32,26 @@ NEXT: H1 (SSRF/injection guard) → C1 (sitemap crawl). Human merge gate: all
   follow-up. Today only $DOMAIN (user-typed) is interpolated. After C1, N
   URLs from a REMOTE sitemap flow into shell commands and fetch targets.
 
+### B1/B2 (Common Crawl backlinks) — KILLED 2026-07-17, measured not assumed
+The plan said Common Crawl was the free backlink source and the 70/100 cap
+was therefore mandatory. Both premises are dead:
+- domain-edges.txt.gz = **17.3 GB gzipped** (+879 MB vertices, +2.3 GB
+  ranks), measured live via HEAD. Finding one domain's inbound links means
+  scanning all of it, per audit. Non-viable, and abusive toward a nonprofit.
+- The implementation everyone cites (claude-seo commoncrawl_graph.py:169)
+  caps at `500 MiB` = **2.9% of the edges file**, and reports what that
+  arbitrary slice held as a backlink profile. A random sample presented as a
+  measurement — the exact failure class this branch exists to remove. We
+  nearly copied it.
+- B2 dies with B1: nothing to cap.
+CONSEQUENCE: I1's narrowed Off-page axis (brand mentions only, backlinks +
+authority declared unauditable in §14) is the FINAL state, not a placeholder.
+Its §14 line was corrected — it used to point at Common Crawl as "nearest
+free source", which is a 17 GB dead end.
+RAISES W2's VALUE: Bing's GetUrlLinks is now the ONLY free viable backlink
+source. First-party only (never a competitor), still blocked on the client's
+Bing account.
+
 ### W2 (Bing) — DEFERRED, blocked on a real-world test
 Killed after 4 challenge rounds. User's model: client sites live on CLIENT
 Bing accounts, so a per-user API key means one key per client account.
@@ -109,12 +131,13 @@ as NEW VERBS. No new architecture.
 - [ ] C3 Internal-link graph — orphan pages + 3-click depth are TODAY stated
       as checks with no command to compute them. C1 unblocks real computation.
 
-### AXE 3 — Off-page real (upgrades I1)
-- [ ] B1 `backlinks` verb — Common Crawl hyperlinkgraph
-      (data.commoncrawl.org/projects/hyperlinkgraph), free, no key.
-- [ ] B2 Honest cap — steal their idea (free-backlink-sources.md:33: cap
-      health at 70/100 when only Common Crawl). Fits our code-ceiling doctrine
-      exactly.
+### AXE 3 — Off-page real (upgrades I1) — SUPERSEDED, see B1/B2 KILLED above
+- [x] ~~B1 `backlinks` verb — Common Crawl hyperlinkgraph~~ KILLED: edges file
+      measured at 17.3 GB gzipped. Non-viable per audit; the reference impl
+      caps at 500 MiB = 2.9% of the graph and calls the remainder a backlink
+      profile.
+- [x] ~~B2 Honest cap at 70/100~~ KILLED with B1: nothing left to cap.
+      I1's narrowed axis is the final state.
 - [ ] B3 VERIFY FIRST: GSC Links API. Subagent claimed "available, OAuth
       already there" — I doubt it: Search Console API v3 has no links endpoint
       (links report is UI-only AFAIK). Verify before planning on it. Do not