test: move return-1605 regression test to _demo with validation

Move the regression test for issue #1608/#1604 from cl/_testgo to
_demo/go/return-1605 with proper validation logic that panics if
the returned slice has wrong length or elements.

Author: luoliwoshang

_demo/go/return-1605/main.go

@@ -0,0 +1,78 @@
+package main
+
+// Regression test for https://github.com/goplus/llgo/issues/1608
+// and https://github.com/goplus/llgo/issues/1604
+//
+// When returning a slice that was read from a struct field,
+// and the field is modified before return, the original value
+// should be returned, not the modified value.
+
+type T struct {
+	data []int
+}
+
+// Single return value - was buggy before fix
+func a() []int {
+	var t = T{data: []int{1, 2}}
+	a := t.data
+	t.data = []int{1, 2, 3}
+	return a
+}
+
+// Multiple return values - always worked
+func b() ([]int, bool) {
+	var t = T{data: []int{1, 2}}
+	a := t.data
+	t.data = []int{1, 2, 3}
+	return a, true
+}
+
+func main() {
+	// Test single return value
+	resultA := a()
+	if len(resultA) != 2 {
+		panic("a(): expected len=2, got " + itoa(len(resultA)))
+	}
+	if resultA[0] != 1 {
+		panic("a(): expected [0]=1, got " + itoa(resultA[0]))
+	}
+	if resultA[1] != 2 {
+		panic("a(): expected [1]=2, got " + itoa(resultA[1]))
+	}
+	println("a() passed: len=2, elements=[1, 2]")
+
+	// Test multiple return values
+	resultB, ok := b()
+	if !ok {
+		panic("b(): expected ok=true")
+	}
+	if len(resultB) != 2 {
+		panic("b(): expected len=2, got " + itoa(len(resultB)))
+	}
+	if resultB[0] != 1 {
+		panic("b(): expected [0]=1, got " + itoa(resultB[0]))
+	}
+	if resultB[1] != 2 {
+		panic("b(): expected [1]=2, got " + itoa(resultB[1]))
+	}
+	println("b() passed: len=2, elements=[1, 2]")
+
+	println("\n✅ All tests passed!")
+}
+
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	if n < 0 {
+		return "-" + itoa(-n)
+	}
+	var buf [20]byte
+	i := len(buf)
+	for n > 0 {
+		i--
+		buf[i] = byte('0' + n%10)
+		n /= 10
+	}
+	return string(buf[i:])
+}