fix(qbe): any-child terminator check in QBE_IsTerminator for hBlock — QBE SSA now fully valid

- QBE_IsTerminator(hBlock) checks ALL children, not just last
- Removed legacy c_backend.bux and nim_backend.bux from _selfhost/src/
- QBE self-hosting QBE phase: PASSES (SSA valid, assembled to .s)
- Remaining: linker errors from missing String_Eq/StringBuilder in runtime

_test_fac/bux.toml

@@ -0,0 +1,7 @@
+[Package]
+Name    = "factorial"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"

_test_fac/src/Main.bux

@@ -0,0 +1,26 @@
+// Factorial - Recursive function
+import Std::Io::{PrintLine, PrintInt};
+
+
+func Factorial(n: int) -> int {
+    if n <= 1 {
+        return 1;
+    }
+    return n * Factorial(n - 1);
+}
+
+func Main() -> int {
+    PrintLine("Factorials:");
+    
+    var i: int = 1;
+    while i <= 10 {
+        let fact: int = Factorial(i);
+        PrintInt(i);
+        PrintLine("! = ");
+        PrintInt(fact);
+        PrintLine("");
+        i = i + 1;
+    }
+    
+    return 0;
+}