feat(verify): store crashing input in FuzzResult for reproducibility

Assisted-by: DeepSeek V4 Pro

verify/fuzz.go

@@ -20,6 +20,7 @@ type FuzzResult struct {
 	Matches    int
 	Mismatches int
 	FirstFail  string // description of the first mismatch ("" if none)
+	CrashInput []byte // input that caused the last crash/mismatch (nil if none)
 }
 
 // OK returns true when all iterations matched.
@@ -30,8 +31,12 @@ func (r FuzzResult) String() string {
 	if r.OK() {
 		return fmt.Sprintf("%s: %d/%d iterations match", r.Func, r.Matches, r.Iterations)
 	}
-	return fmt.Sprintf("%s: %d/%d match, %d MISMATCH — %s",
+	s := fmt.Sprintf("%s: %d/%d match, %d MISMATCH — %s",
 		r.Func, r.Matches, r.Iterations, r.Mismatches, r.FirstFail)
+	if len(r.CrashInput) > 0 {
+		s += fmt.Sprintf("\n  input: %x", r.CrashInput)
+	}
+	return s
 }
 
 // funcSig is a parsed // func signature from the assembly source.
@@ -160,6 +165,9 @@ func (k *Kernel) FuzzFunc(name string, sig funcSig, goCode []byte, iterations in
 		// (e.g. histogram increments) don't corrupt the other's input.
 		gasmArgs, goArgs, bufs := genDualArgs(rng, sig, fl.Args)
 
+		// Save the current input for crash diagnostics.
+		result.CrashInput = gasmArgs
+
 		// Call the gasm version.
 		gasmOut, err := k.CallFunc(name, gasmArgs)
 		if err != nil {