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/prime.c

@@ -0,0 +1,28 @@
+#include <stdio.h>
+
+main() {
+	int n;
+	int t;
+	int c;
+	int p;
+
+	c = 0;
+	n = 2;
+	while (n < 5000) {
+		t = 2;
+		p = 1;
+		while (t*t <= n) {
+			if (n % t == 0)
+				p = 0;
+			t++;
+		}
+		if (p) {
+			if (c && c % 10 == 0)
+				printf("\n");
+			printf("%4d ", n);
+			c++;
+		}
+		n++;
+	}
+	printf("\n");
+}