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/dark.ssa

@@ -0,0 +1,32 @@
+# skip arm64 arm64_apple rv64
+# a hack example,
+# we use a dark type to get
+# a pointer to the stack.
+
+type :magic = align 1 { 0 }
+
+data $ret = { l 0 }
+
+export
+function $test(:magic %p) {
+@start
+	%av =w loadw $a
+	%a1 =w add 1, %av
+	storew %a1, $a       # increment $a
+	%r1 =l loadl $ret    # fetch from $ret
+	%p1 =l add %p, -8
+	%r2 =l loadl %p1     # get the return address
+	storel %r2, $ret     # store it in $ret
+	%c =w ceql %r1, %r2
+	jnz %c, @fin, @cal
+@cal
+	%i =w call $test()   # no argument given, intentionally!
+@fin
+	ret
+}
+
+# >>> driver
+# extern void test(void);
+# int a = 2;
+# int main() { test(); return !(a == 5); }
+# <<<