fix(examples): 0712 sha256 streaming writes scratch to .sx-tmp not /tmp [FINAL-rem F2]

Scratch file path violated the project rule 'scratch files go in
.sx-tmp/, never /tmp'. Route the temp .bin through the repo-local,
gitignored .sx-tmp/ dir, creating it at runtime via create_dir_all
(mirroring 0713) so the test is self-contained on a clean checkout.
Digest assertions and output are unchanged.

examples/0712-modules-sha256-streaming.sx

@@ -59,7 +59,8 @@ main :: () {
     check("split-on-boundary", stream_split(MSG, 64), PIN);   // 64: exact block edge
 
     // sha256_file (streaming) must equal the in-memory digest.
-    path := "/tmp/sx_0712_stream.bin";
+    if !create_dir_all(".sx-tmp") { print("mkdir: FAIL\n"); return; }
+    path := ".sx-tmp/sx_0712_stream.bin";
     if !write_file(path, MSG) { print("file-write: FAIL\n"); return; }
 
     maybe := sha256_file(path);