feat(seo-data): C2 — cannibalisation from Google's own data, one param away

The inventory called this "no duplicate-content / cannibalisation detection".
Splitting that into its two halves shows one is free and the other is a trap.

CANNIBALISATION — free, and the data was already reachable. Search Analytics
has always accepted several dimensions at once ("no limit to the number of
dimensions that you can group by"); this engine only ever sent
`"dimensions": [dim]` and _norm_queries only ever read `keys[0]`. So
query+page — the pairing that exposes the conflict — was one parameter away
and nobody asked. Same shape of win as W1.

  fetch.sh cannibal → queries where 2+ of OUR pages compete, ranked by total
  impressions, strongest page first inside each. Same auth, same quota family,
  no new scope. `capped` reports a full row window rather than presenting a
  truncated list as exhaustive — same rule as COVERAGE and the sitemap caps.

Grouping happens in the engine, deterministically: asking an LLM to group
1000 rows by query is arithmetic it should never be handed.

Backward compatible: rows gained `keys` (the list the API actually returns);
`key` stays as keys[0], so the single-dim quick-wins consumer is untouched.
A test pins both.

30/70 DUPLICATION — deliberately NOT built, and this is the honest half.
Measuring it needs main-content extraction (strip nav/header/footer). Without
that, comparing two same-template pages returns ~95% similar for every site —
a confident false positive, which is exactly the failure class the rest of
this branch exists to remove. It stays an explicit LLM judgement over the >=3
same-family pages C1c now samples for it, labelled as judgement, never quoting
a similarity percentage nobody computed. A wrong number would be worse than
the current honest gap.

The two must not be merged in the report either: cannibalisation is a SERP
fact Google measured; 30/70 is a content question. The spec now says so.

Verified: fixture with 3 pages on one query, 2 on another, 1 on a third →
2 conflicts, correct ranking, single-page query excluded; live dispatch
degrades cleanly with no account; seo-data 110 -> 119 pass, 0 fail; full suite
green; shellcheck + py_compile clean.

lib/seo-data/fixtures-cannibal/gsc_queries.json

@@ -0,0 +1,7 @@
+{"rows":[
+  {"keys":["plombier paris","https://ex.com/plombier"],"clicks":40,"impressions":900,"ctr":0.044,"position":6.3},
+  {"keys":["plombier paris","https://ex.com/services/plomberie"],"clicks":3,"impressions":300,"ctr":0.010,"position":14.1},
+  {"keys":["urgence fuite","https://ex.com/urgence"],"clicks":5,"impressions":1200,"ctr":0.004,"position":8.9},
+  {"keys":["urgence fuite","https://ex.com/blog/fuite-que-faire"],"clicks":2,"impressions":800,"ctr":0.003,"position":11.4},
+  {"keys":["urgence fuite","https://ex.com/services/depannage"],"clicks":1,"impressions":400,"ctr":0.002,"position":19.2},
+  {"keys":["devis plomberie","https://ex.com/devis"],"clicks":9,"impressions":150,"ctr":0.060,"position":4.1}]}