feat(simulation): add the overflow rule, a one-sided gap for hard ceilings

A fourth Rule kind: overflow = max(0, factor × (level − threshold)). It stays
shut until its `+` level passes its `−` threshold, so an outflow on it spills only
the excess — a spillway / hard ceiling the smooth rules can't draw.

Gap can't stand in for it: it's signed and bidirectional (coffee cooling relies on
that), so below the threshold a gap outflow runs backwards. Clamping at zero is the
whole point, and it can't be applied globally without breaking gap. This is the
additive vocabulary growth ADR-0004 anticipates — a new kind in the union plus a
case in the evaluator — alongside touch-ups to the rule validator and the inspector.

src/components/Inspector.vue

@@ -115,6 +115,8 @@ const RULE_HINT: Record<Rule["kind"], string> = {
   constant: "A fixed number — no inputs.",
   proportional: "rate = factor × its “+” inputs.",
   gap: "rate = factor × (level − target): the “+” input is the level, the “−” the target.",
+  overflow:
+    "rate = max(0, factor × (level − threshold)): spills only once the “+” level passes the “−” threshold.",
 }
 </script>
 
@@ -167,6 +169,7 @@ const RULE_HINT: Record<Rule["kind"], string> = {
             <option value="constant">Constant</option>
             <option value="proportional">Proportional</option>
             <option value="gap">Gap</option>
+            <option value="overflow">Overflow</option>
           </select>
         </label>