fix: Replace unreliable memory pressure check with swap indicator

The previous `major_faults > 10` check is unreliable on macOS, as it counts file-backed mmap page-ins rather than true memory pressure. This change replaces it with `swaps > 0`, a more accurate cross-platform indicator of RAM exhaustion. The swap diagnosis message is also updated to clarify that the working set exceeds available RAM, and comments are added to document this rationale.

src/obisys/src/lib.rs

@@ -117,24 +117,20 @@ fn diagnose(s: &StageStats, n_cores: usize) -> Diagnosis {
     let cpu_pct = eff * 100.0;
     let io_ops  = s.in_blocks + s.out_blocks;
 
+    // swaps > 0 is the only reliable cross-platform indicator of true RAM exhaustion.
+    // ru_majflt is intentionally excluded: on macOS it counts all file-backed mmap
+    // page-ins (even from page cache), making it useless as a memory-pressure signal
+    // for mmap-heavy code. On Linux it is more meaningful, but swaps covers the
+    // severe case on both platforms.
     if s.swaps > 0 {
         return Diagnosis {
             tag: "swapping",
             detail: Some(format!(
-                "swapped {} time(s) — severe memory pressure, consider increasing RAM",
+                "swapped {} time(s) — working set exceeds available RAM",
                 s.swaps,
             )),
         };
     }
-    if s.major_faults > 10 {
-        return Diagnosis {
-            tag: "mem pressure",
-            detail: Some(format!(
-                "{} major page faults — working set exceeds available RAM",
-                s.major_faults,
-            )),
-        };
-    }
     if eff < 0.3 && io_ops > 100 {
         return Diagnosis {
             tag: "disk I/O",