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/test/puts10.ssa

@@ -0,0 +1,30 @@
+export
+function $main() {
+@start
+	%y  =l alloc4 4
+	%y1 =l add %y, 1
+	storeb 0, %y1
+@loop
+	%n =w phi @start 0, @loop %n1
+	%c =w add %n, 48
+	storeb %c, %y
+	%r =w call $puts(l %y)
+	%n1 =w add %n, 1
+	%cmp =w cslew %n1, 9
+	jnz %cmp, @loop, @end
+@end
+	ret
+}
+
+# >>> output
+# 0
+# 1
+# 2
+# 3
+# 4
+# 5
+# 6
+# 7
+# 8
+# 9
+# <<<