feat(demo): add unique package demo from issue #1358

Added _demo/go/unique with code demonstrating unique.Make usage.
This demo showcases the fix for the compilation panic issue.

Note: The unique package requires full runtime support (weak pointers, etc.)
which is still under development. The test_demo.sh script should be
updated separately to skip this demo on Go 1.23 or earlier.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <cpunion@users.noreply.github.com>

_demo/go/unique/main.go

@@ -0,0 +1,38 @@
+package main
+
+import "unique"
+
+func main() {
+	var h1 = unique.Make(int(42))
+	var h2 = unique.Make(int(42))
+	if h1 != h2 {
+		panic("h1 and h2 should be equal")
+	}
+	var v1 = h1.Value()
+	var v2 = h2.Value()
+	if v1 != v2 || v1 != 42 {
+		panic("values should be equal to 42")
+	}
+
+	var h3 = unique.Make("hello")
+	var h4 = unique.Make("hello")
+	if h3 != h4 {
+		panic("h3 and h4 should be equal")
+	}
+	var s1 = h3.Value()
+	var s2 = h4.Value()
+	if s1 != s2 || s1 != "hello" {
+		panic("values should be equal to 'hello'")
+	}
+
+	var h5 = unique.Make(int(100))
+	var h6 = unique.Make(int(200))
+	if h5 == h6 {
+		panic("h5 and h6 should not be equal")
+	}
+	var n1 = h5.Value()
+	var n2 = h6.Value()
+	if n1 != 100 || n2 != 200 {
+		panic("values should be 100 and 200 respectively")
+	}
+}