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/max.ssa

@@ -0,0 +1,34 @@
+# find the maximum value
+# in a nul-terminated array
+# of unsigned bytes
+#
+# the output is stored in $a
+
+data $arr = { b 10, b -60, b 10, b 100, b 200, b 0 }
+
+export
+function $test() {
+@start
+@loop
+	%max =w phi @start -1, @new %byt, @old %max
+	%loc =l phi @start $arr,  @new %loc1, @old %loc1
+	%byt =w loadub %loc
+	%loc1 =l add 1, %loc
+	jnz %byt, @iter, @end
+@iter
+	%cmp =w cslew %max, %byt
+	jnz %cmp, @new, @old
+@new
+	jmp @loop
+@old
+	jmp @loop
+@end
+	storew %max, $a
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a;
+# int main() { test(); return !(a == 200); }
+# <<<