driver-manager: N20 audit - log silent heartbeat write failures (S6)

Replace three silently-dropped Result cases in heartbeat::write_atomic
(create_dir_all, fs::write, fs::rename) with log::error! diagnostics.
Brings heartbeat.rs in line with the other 11 modules in the crate
(172 existing log::* sites) and the project WARNING POLICY.

Also removes the orphaned temp file when rename fails (e.g. cross-fs
EXDEV) so leftover .tmp files do not accumulate across restarts.

Success-path behavior is unchanged; the 3 existing heartbeat unit
tests still pass.

local/recipes/system/driver-manager/source/src/heartbeat.rs

@@ -98,14 +98,43 @@ impl Heartbeat {
 fn write_atomic(path: &Path, bytes: &[u8]) {
     if let Some(parent) = path.parent() {
         if !parent.as_os_str().is_empty() {
-            let _ = fs::create_dir_all(parent);
+            // N20 S6: log create_dir_all failure (was silently dropped).
+            if let Err(e) = fs::create_dir_all(parent) {
+                log::error!(
+                    "heartbeat: create_dir_all({}) failed: {}",
+                    parent.display(),
+                    e
+                );
+                return;
+            }
         }
     }
     let tmp = path.with_extension("tmp");
-    if fs::write(&tmp, bytes).is_err() {
+    // N20 S6: log write failure (was silently dropped). Includes
+    // disk-full, read-only-fs, permission-denied. Heartbeat-thread
+    // continues running (heartbeat is best-effort) but operators now
+    // see the actual cause in the log.
+    if let Err(e) = fs::write(&tmp, bytes) {
+        log::error!(
+            "heartbeat: write {} ({} bytes) failed: {}",
+            tmp.display(),
+            bytes.len(),
+            e
+        );
         return;
     }
-    let _ = fs::rename(&tmp, path);
+    // N20 S6: log rename failure (was silently dropped). Cross-fs
+    // rename (e.g. tmpfs → /var/run on different fs) returns EXDEV.
+    // The temp file is left behind but a subsequent write will overwrite.
+    if let Err(e) = fs::rename(&tmp, path) {
+        log::error!(
+            "heartbeat: rename {} -> {} failed: {}",
+            tmp.display(),
+            path.display(),
+            e
+        );
+        let _ = fs::remove_file(&tmp);
+    }
 }
 
 /// Cheap cloneable handle that the manager uses to bump counters.