Fix NVMe serial display and improve pool health logic

- Fix physical drive serial number display in dashboard
- Improve pool health calculation for arrays with multiple disks
- Support proper tree symbols for multiple parity drives
- Read git commit hash from /var/lib/cm-dashboard/git-commit for Build display

agent/src/collectors/disk.rs

@@ -639,10 +639,19 @@ impl DiskCollector {
         }).collect();
         
         // Calculate overall pool health string and status
-        let (pool_health, health_status) = match (failed_data, failed_parity) {
-            (0, 0) => ("healthy".to_string(), cm_dashboard_shared::Status::Ok),
-            (1, 0) | (0, 1) => ("degraded".to_string(), cm_dashboard_shared::Status::Warning),
-            _ => ("critical".to_string(), cm_dashboard_shared::Status::Critical),
+        // SnapRAID logic: can tolerate up to N parity drive failures (where N = number of parity drives)
+        // If data drives fail AND we've lost parity protection, that's critical
+        let (pool_health, health_status) = if failed_data == 0 && failed_parity == 0 {
+            ("healthy".to_string(), cm_dashboard_shared::Status::Ok)
+        } else if failed_data == 0 && failed_parity > 0 {
+            // Parity failed but no data loss - degraded (reduced protection)
+            ("degraded".to_string(), cm_dashboard_shared::Status::Warning)
+        } else if failed_data == 1 && failed_parity == 0 {
+            // One data drive failed, parity intact - degraded (recoverable)
+            ("degraded".to_string(), cm_dashboard_shared::Status::Warning)
+        } else {
+            // Multiple data drives failed OR data+parity failed = data loss risk
+            ("critical".to_string(), cm_dashboard_shared::Status::Critical)
         };
         
         // Calculate pool usage status using config thresholds

agent/src/collectors/nixos.rs

@@ -83,14 +83,25 @@ impl NixOSCollector {
         std::env::var("CM_DASHBOARD_VERSION").unwrap_or_else(|_| "unknown".to_string())
     }
 
-    /// Get NixOS system generation (build) information
+    /// Get NixOS system generation (build) information from git commit
     async fn get_nixos_generation(&self) -> Option<String> {
-        match Command::new("nixos-version").output() {
-            Ok(output) => {
-                let version_str = String::from_utf8_lossy(&output.stdout);
-                Some(version_str.trim().to_string())
+        // Try to read git commit hash from file written during rebuild
+        let commit_file = "/var/lib/cm-dashboard/git-commit";
+        match fs::read_to_string(commit_file) {
+            Ok(content) => {
+                let commit_hash = content.trim();
+                if commit_hash.len() >= 7 {
+                    debug!("Found git commit hash: {}", commit_hash);
+                    Some(commit_hash.to_string())
+                } else {
+                    debug!("Git commit hash too short: {}", commit_hash);
+                    None
+                }
+            }
+            Err(e) => {
+                debug!("Failed to read git commit file {}: {}", commit_file, e);
+                None
             }
-            Err(_) => None,
         }
     }
 }