feat(routing)!: task rules become per-complexity model maps

The single per-task «Complexity» pin was the wrong shape — what you actually
decide is WHICH model serves a task at WHICH complexity. A task rule is now a
per-tier map, edited in the console as rows of “model ▾ + complexity ▾” pairs
(plus ×/add) inside each task:

  [routing.tasks.code]
  low  = ["cheap-coder"]
  high = ["best-coder"]
  any  = ["generic-coder"]   # leads at every tier after the tier pick

Chain order: the task's tier-specific list → (at high: the global high chain,
so escalation still beats any-tier pins) → the task's any-tier list → the
global tier chain → default as the last resort; deduplicated. Plain-array
rules keep meaning "any tier" unchanged, and the short-lived {models, tier}
form from the previous commit is normalized into the map on load.

Verified live: legacy config migrated visibly in the editor, pairs survive
save+reload, a math prompt at low ignored the math@high pair, per-tier leads
confirmed by unit tests (19 passing).

web/js/i18n/fr.js

@@ -626,5 +626,7 @@ export default {
   "keys.col.costAll": "$ au total",
   "routing.tasks.tier": "Complexité",
   "routing.tasks.tierAuto": "Auto (routeur)",
-  "routing.tasks.tierHint": "Fixer la complexité de cette tâche : le verdict du routeur est remplacé, high continue d'escalader au-dessus de la liste de modèles.",
+  "routing.tasks.tierHint": "Quel modèle sert cette tâche à quelle complexité ; « toute » mène à tous les niveaux. L'ordre = ordre de repli. Un verdict high escalade toujours au-dessus des entrées « toute ».",
+  "routing.tasks.tierAny": "toute complexité",
+  "routing.tasks.addModel": "modèle",
 };