chore: web_diag keeps its doc comment; sentinel marker written with \n

#1521 inserted crash_log between web_diag's doc comment and its module, so the comment documented crash_log instead. #1519 wrote the survived marker with a literal line break inside the string.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gpu_backend.rs

@@ -345,8 +345,7 @@ pub fn mark_sentinel_survived() {
     if text.lines().any(|l| l.trim() == SURVIVED_MARKER) {
         return;
     }
-    let _ = std::fs::write(&p, format!("{text}{SURVIVED_MARKER}
-"));
+    let _ = std::fs::write(&p, format!("{text}{SURVIVED_MARKER}\n"));
 }
 
 pub fn arm_sentinel(backend: &str) {