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.

lib/seo-data/seo-data.test.sh

@@ -178,6 +178,35 @@ has "cap is reported"            "$CAP" '"capped": true'
 has "capped withholds orphans"   "$CAP" '"orphans_withheld": true'
 hasnt "capped emits no orphans"  "$CAP" '"orphans":'
 
+echo "── score (I7) ──"
+sc() { printf '%s' "$1" | python3 "$SD/score.py" --findings -; }
+# technical: haute(-8) + moyenne(-3) = 100-11 = 89 → 17.8
+B='{"depth":"FULL","profile":"local","axes":{"technical":{"findings":[{"severity":"haute"},{"severity":"moyenne"}]},"seo-local":{"findings":[]},"off-page":{"findings":[]},"social":{"findings":[]},"competitive":{"findings":[]},"legal":{"findings":[]},"on-page":{"findings":[]}}}'
+R="$(sc "$B")"
+has "harden scale, /5 into /20"   "$R" '"score_20": 17.8'
+has "no findings = 20"            "$R" '"score_20": 20.0'
+has "nothing renormalised"        "$R" '"weights_renormalised": false'
+# THE point of I7: same findings in, same score out
+A1="$(sc "$B" | python3 -c 'import sys,json;print(json.load(sys.stdin)["global_20"])')"
+A2="$(sc "$B" | python3 -c 'import sys,json;print(json.load(sys.stdin)["global_20"])')"
+[ "$A1" = "$A2" ] && ok "score is reproducible" || no "score is reproducible" "$A1 vs $A2"
+# N/A is not a zero, and R2 mandated renormalising by hand — now computed
+NA='{"depth":"FULL","profile":"local","axes":{"technical":{"findings":[]},"on-page":{"status":"na"},"seo-local":{"findings":[]},"off-page":{"status":"na"},"social":{"findings":[]},"competitive":{"findings":[]},"legal":{"findings":[]}}}'
+RN="$(sc "$NA")"
+has "na axes listed"             "$RN" '"on-page"'
+has "renormalisation flagged"    "$RN" '"weights_renormalised": true'
+# all axes 20 → global must stay 20: N/A must not drag the mean down
+has "na is not a zero"           "$RN" '"global_20": 20.0'
+# prevalence shifts severity ONE step, both ways
+WIDE='{"depth":"LOCAL","profile":"local","axes":{"technical":{"findings":[{"severity":"moyenne","affected":10,"sampled":12}]},"on-page":{"findings":[]},"seo-local":{"findings":[]},"legal":{"findings":[]}}}'
+ONE='{"depth":"LOCAL","profile":"local","axes":{"technical":{"findings":[{"severity":"moyenne","affected":1,"sampled":12}]},"on-page":{"findings":[]},"seo-local":{"findings":[]},"legal":{"findings":[]}}}'
+has "widespread escalates (-8)"  "$(sc "$WIDE")" '"score_20": 18.4'
+has "isolated de-escalates (-1)" "$(sc "$ONE")"  '"score_20": 19.8'
+# malformed input is an error, never a silently wrong number
+has "unknown severity rejected"  "$(sc '{"depth":"FULL","profile":"local","axes":{"technical":{"findings":[{"severity":"bogus"}]}}}')" '"status": "error"'
+has "unknown profile rejected"   "$(sc '{"depth":"FULL","profile":"martian","axes":{}}')" '"status": "error"'
+has "garbage json is an error"   "$(sc 'not json')" '"status": "error"'
+
 echo "── drift (H2) ──"
 DH="$(mktemp -d)"
 D1="$(HOME="$DH" SEO_DATA_MOCK_DIR="$SD/fixtures-drift-v1" python3 "$SD/drift.py" \