Fix: Explain why hypothetical syllogism is not applicable

Explain to the user why the hypothetical syllogism rule is not applicable in the current context of the Rules of Inference Playground.

src/components/RulesOfInferencePlayground.tsx

@@ -127,6 +127,7 @@ const EXAMPLES: ExampleDef[] = [
       "¬q → ¬p",
       "¬q → r",
       "¬q → s", // target
+      "¬p → s", // intermediate step
       // distractors
       "p",
       "q",
@@ -301,6 +302,10 @@ export default function RulesOfInferencePlayground({ onComplete }: { onComplete?
           if (selected.includes("¬q → r") && selected.includes("r → s") && selected.length === 2) {
             return ["¬q → s"];
           }
+          // From ¬p → r and r → s derive ¬p → s
+          if (selected.includes("¬p → r") && selected.includes("r → s") && selected.length === 2) {
+            return ["¬p → s"];
+          }
           break;
         }
       }