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_strings/src/Main.bux

@@ -0,0 +1,34 @@
+// Strings - String manipulation with Std::String
+import Std::Io::{PrintLine, PrintInt};
+import Std::String::{String_Len, String_Eq, String_Concat};
+
+func Main() -> int {
+    let hello: String = "Hello";
+    let world: String = "World";
+    
+    PrintLine("String operations:");
+    
+    // Length
+    PrintLine("Length of 'Hello':");
+    PrintInt(String_Len(hello));
+    PrintLine("");
+    
+    // Concatenation
+    let greeting: String = String_Concat(hello, ", ");
+    let greeting2: String = String_Concat(greeting, world);
+    let full: String = String_Concat(greeting2, "!");
+    
+    PrintLine("Concatenated:");
+    PrintLine(full);
+    
+    // Equality
+    if String_Eq(hello, "Hello") {
+        PrintLine("'Hello' equals 'Hello'");
+    }
+    
+    if !String_Eq(hello, world) {
+        PrintLine("'Hello' does not equal 'World'");
+    }
+    
+    return 0;
+}