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

vendor/qbe/minic/test/euler9.c

@@ -0,0 +1,27 @@
+#include <stdio.h>
+
+main()
+{
+	int i;
+	int a;
+	int b;
+	int c;
+	int d;
+	
+	for (a = 1; a < 1000; a++) {
+		for (b = a + 1; b < 1000; b++) {
+			d = a*a + b*b;
+			for (i = 0; i < 1000; i++) {
+				if (i * i == d) {
+					c = i;
+					if (b < c && a+b+c == 1000) {
+						printf("%d\n", a*b*c);
+						return 0;
+					}
+					break;
+				}
+			}
+		}
+	}
+}
+