test(metrics): match process assertions to collector platforms

Darwin does not register the Linux ProcessCollector, so the repository-counting test must not require its CPU and resident-memory samples. Keep those assertions inside the existing Linux block while retaining repository metrics coverage on every platform.

This changes only platform-specific test expectations; production metrics and Linux coverage remain unchanged. Validation: inspected the collector registration guard and checked the diff. Darwin execution is still required on a Darwin worker.

Assisted-by: Codex (GPT-6)

src/metrics/mod.rs

@@ -1216,10 +1216,12 @@ mod tests {
         // render() recounts from disk)
         let output = metrics.render();
         assert!(output.contains("ngit_repositories_total 0"));
-        assert!(output.contains("process_cpu_seconds_total"));
-        assert!(output.contains("process_resident_memory_bytes"));
         #[cfg(target_os = "linux")]
         {
+            // ProcessCollector is registered only on Linux, just like the
+            // cgroup collectors below. Repository metrics are portable.
+            assert!(output.contains("process_cpu_seconds_total"));
+            assert!(output.contains("process_resident_memory_bytes"));
             if let Some(directory) = cgroup_v2_directory() {
                 if cgroup_cpu_seconds(&directory).is_some() {
                     assert!(output.contains("ngit_cgroup_cpu_seconds_total"));