feat: execute conditionals through typed IR

Use typed singleton arrays for braceless arm blocks: &[statement] compiled but trapped under the Bro bootstrap.

programs/factorial.hon

@@ -0,0 +1,8 @@
+factorial proc(n int) int {
+    if (n <= 1) return 1
+    return n * factorial(n - 1)
+}
+
+main proc() int {
+    return factorial(5)
+}