feat(seo-data): I7 — compute the score instead of feeling it

/harden has a real scale (SKILL.md:435 — Critique -15, Haute -8, Moyenne -3,
Basse -1, clamp [0,100]). /seo had none: every axis was felt, so two runs over
identical code could disagree. That is a credibility problem on its own, and
/client-handover gates on 17/20 — a wobbling number makes the gate arbitrary.
H2 sharpened it: now that drift reports what actually changed, a score moving
on its own is visibly noise.

The split is the whole point. WHICH findings exist and how severe each is
stays the LLM's judgement — irreducible, and I am not pretending otherwise.
The arithmetic stops being judgement: same findings in, same score out. Same
principle as grouping cannibalisation rows in the engine rather than handing a
model 1000 rows to add up.

Reuses /harden's scale, /5 into /20, so the family speaks one vocabulary
instead of two.

Two things it makes real that were prose:
- **N/A is not a zero.** R2 (client-rendered on-page) and I1 (unauditable
  off-page) both mandate excluding an axis and renormalising the rest. Both
  left that arithmetic to the model. Now the engine does it and refuses to let
  N/A behave like a zero — verified: all-20 axes with two N/A still yields
  global 20.0, not a dragged-down mean.
- **Prevalence.** affected/sampled shift severity ONE step (>=50% escalates, a
  single page de-escalates). A defect on 1 of 12 pages is not the defect on
  12 of 12, and flattening the two is part of what made the old numbers move.

Malformed input is an error, never a silently wrong number — unlike the fetch
verbs, a degrade here would mean bad input, not a network fact. Unknown
severity and unknown profile both rejected, tested.

Verified: hand-checkable arithmetic (haute+moyenne = 100-11 = 89 → 17.8;
critique+haute = 77 → 15.4), identical global across repeated runs, weights
renormalised to sum 1.0 with two axes N/A. seo-data 155 -> 167 pass, 0 fail;
full suite green; shellcheck + py_compile clean.

agents/seo-analyzer.md

@@ -898,6 +898,41 @@ FIX: AUTO (<what agent will do>) | USER (<what user must do>)
 | Competitive position | 5% | 10% | |
 | Legal compliance | 10% | 5% | |
 
+**Compute the scores, do not feel them (I7).** Emit your findings, then let
+the engine do the arithmetic:
+
+```bash
+bash ~/.claude/lib/seo-data/fetch.sh score --findings /tmp/seo-findings.json
+```
+
+```json
+{"depth":"FULL","profile":"local",
+ "axes":{"technical":{"findings":[{"severity":"haute","affected":9,"sampled":12}]},
+         "on-page":{"status":"na","reason":"client-rendered (R2)"},
+         "off-page":{"status":"na","reason":"backlinks unauditable (I1)"}}}
+```
+
+`profile`: `local` (B2C) | `national` (SaaS/national/content). Severities are
+`critique|haute|moyenne|basse` — `/harden`'s scale (-15/-8/-3/-1, clamp,
+then /5 into /20), so the whole skill family speaks one vocabulary.
+
+**The split matters.** WHICH findings exist and how severe each is stays your
+judgement — irreducible. The addition is not: same findings in, same score
+out. Until now every axis was felt, so two runs over identical code could
+disagree, and `/client-handover` gates on 17/20.
+
+- `affected`/`sampled` (optional) shift severity ONE step: ≥50% of the sample
+  escalates, a single page de-escalates. A defect on 1 of 12 pages is not the
+  defect on 12 of 12; pretending so is what made the old numbers wobble.
+- `status: "na"` → the axis is EXCLUDED and the remaining weights are
+  renormalised for you. This is the R2 rule (client-rendered on-page) and the
+  I1 rule (unauditable off-page), finally computed instead of done by hand.
+  **N/A is not a zero** and the engine will not let it behave like one.
+- `status: "error"` → malformed findings. Fix them; never fall back to
+  eyeballing a number.
+- Run it twice on the same file before publishing. If the output moved, your
+  findings moved, and that is the thing to explain.
+
 **Technical axis note:** CWV scored on CrUX field data (75th percentile,
 real users, from STEP 4) when available; otherwise lab PageSpeed
 Lighthouse run.