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_structs/src/Main.bux

@@ -0,0 +1,29 @@
+// Structs - Basic struct usage
+import Std::Io::{PrintLine, PrintInt};
+
+
+struct Point {
+    x: int;
+    y: int;
+}
+
+func AddPoints(a: Point, b: Point) -> Point {
+    let result: Point = Point { x: a.x + b.x, y: a.y + b.y };
+    return result;
+}
+
+func Main() -> int {
+    let p1: Point = Point { x: 10, y: 20 };
+    let p2: Point = Point { x: 5, y: 15 };
+    let sum: Point = AddPoints(p1, p2);
+    
+    PrintLine("Point sum:");
+    PrintLine("x = ");
+    PrintInt(sum.x);
+    PrintLine("");
+    PrintLine("y = ");
+    PrintInt(sum.y);
+    PrintLine("");
+    
+    return 0;
+}