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/fetch.sh

@@ -32,6 +32,8 @@ case "$cmd" in
   # No auth, no Google: stdlib-only, runs even without the venv.
   sitemap)
     exec "$PY" "$HERE/sitemap.py" --store "$STORE" "$@" ;;
+  score)
+    exec "$PY" "$HERE/score.py" --store "$STORE" "$@" ;;
   drift)
     exec "$PY" "$HERE/drift.py" --store "$STORE" "$@" ;;
   rendercheck)
@@ -50,6 +52,6 @@ case "$cmd" in
     fi
     echo '{"status":"error","reason":"usage: fetch.sh forget {--label <label>|--all} (label charset: A-Za-z0-9._-)"}'
     exit 2 ;;
-  *) echo '{"status":"error","reason":"usage: fetch.sh {accounts|crux|queries|inspect|cannibal|sitemap|rendercheck|linkgraph|drift|forget} [flags]"}'
+  *) echo '{"status":"error","reason":"usage: fetch.sh {accounts|crux|queries|inspect|cannibal|sitemap|rendercheck|linkgraph|drift|score|forget} [flags]"}'
      exit 2 ;;
 esac