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_fib/bux.toml

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

_test_fib/src/Main.bux

@@ -0,0 +1,24 @@
+// Fibonacci - Recursive function with while loop
+import Std::Io::{PrintLine, PrintInt};
+
+
+func Fibonacci(n: int) -> int {
+    if n <= 1 {
+        return n;
+    }
+    return Fibonacci(n - 1) + Fibonacci(n - 2);
+}
+
+func Main() -> int {
+    PrintLine("Fibonacci sequence:");
+    
+    var i: int = 0;
+    while i < 10 {
+        let fib: int = Fibonacci(i);
+        PrintInt(fib);
+        PrintLine("");
+        i = i + 1;
+    }
+    
+    return 0;
+}