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/css/styles.css

@@ -592,8 +592,10 @@ body.modal-open { overflow: hidden; }
 
 .ok { color: var(--good); }
 
-.task-tier-wrap { display: flex; align-items: center; gap: 6px; margin-left: 10px; white-space: nowrap; }
-.task-tier { max-width: 130px; padding: 4px 8px; font-size: 12px; }
+.pairs { display: flex; flex-direction: column; gap: 6px; align-items: flex-start; }
+.pair-row { display: flex; gap: 6px; align-items: center; }
+.pair-model { min-width: 220px; max-width: 340px; padding: 4px 8px; font-size: 12.5px; }
+.pair-tier { max-width: 150px; padding: 4px 8px; font-size: 12.5px; }
 
 /* Benchmark view */
 @keyframes bm-in { from { opacity: 0; transform: translateY(-3px); } to { opacity: 1; transform: none; } }