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/double.ssa

@@ -0,0 +1,25 @@
+export
+function $test() {
+@start
+	%x1 =d copy d_0.1
+	%x2 =d add d_0.2, %x1
+	%x3 =d sub %x2, d_0.3
+
+@loop
+	%x4 =d phi @start %x3, @loop %x5
+	%i1 =w phi @start 0, @loop %i2
+	%x5 =d add %x4, %x4
+	%i2 =w add %i1, 1
+	%c0 =w cled %x5, 4607182418800017408 # d_1.0
+	jnz %c0, @loop, @end
+
+@end
+	storew %i2, $a
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a;
+# int main() { test(); return !(a == 55); }
+# <<<