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/knight.c

@@ -0,0 +1,60 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+void *calloc();
+
+int N;
+int **b;
+
+board()
+{
+	int x;
+	int y;
+
+	for (y=0; y<8; y++) {
+		for (x=0; x<8; x++)
+			printf(" %02d", b[x][y]);
+		printf("\n");
+	}
+	printf("\n");
+	return 0;
+}
+
+chk(int x, int y)
+{
+	if (x < 0 || x > 7 || y < 0 || y > 7)
+		return 0;
+	return b[x][y] == 0;
+}
+
+go(int k, int x, int y)
+{
+	int i;
+	int j;
+
+	b[x][y] = k;
+	if (k == 64) {
+		if (x != 2 && y != 0 && abs(x-2) + abs(y) == 3) {
+			board();
+			N++;
+			if (N == 10)
+				exit(0);
+		}
+	} else
+		for (i=-2; i<=2; i++)
+			for (j=-2; j<=2; j++) 
+				if (abs(i) + abs(j) == 3 && chk(x+i, y+j)) 
+					go(k+1, x+i, y+j);
+	b[x][y] = 0;
+	return 0;
+}
+
+main()
+{
+	int i;
+
+	b = calloc(8, sizeof (int *));
+	for (i=0; i<8; i++)
+		b[i] = calloc(8, sizeof (int));
+	go(1, 2, 0);
+}