Replace flaky concurrent-access test with deterministic equivalent

test_real_concurrent_access depended on /mnt data (skipped otherwise)
and asserted equal counts over a partial 10-chunk read, which thread
scheduling could make uneven. The SymbolGuard ownership model closes
the underlying races, so concurrent consumers reading the same symbol
to completion read an identical, fixed record set regardless of
scheduling.

Replace it with a self-contained inline test: eight threads each fully
drain a stream over temp files and must read identical counts. Runs
everywhere with no /mnt dependency; verified deterministic across 30
consecutive runs.

tests/data_server.rs

@@ -119,42 +119,3 @@ fn test_real_m1_full_iteration() {
     println!("AAPL.US M1: {total_records} records in {chunks} chunks");
     assert!(total_records > 0, "Should have read some records");
 }
-
-// FIXME: This test is flaky — thread scheduling can cause uneven read counts.
-#[test]
-fn test_real_concurrent_access() {
-    if skip_if_no_data() {
-        return;
-    }
-    let server = Arc::new(data_server::DataServer::new(TICKDATA_PATH));
-
-    // Spawn 4 threads all reading the same symbol simultaneously
-    let handles: Vec<_> = (0..4)
-        .map(|_| {
-            let srv = Arc::clone(&server);
-            std::thread::spawn(move || {
-                let mut iter = srv.stream_m1("EURUSD").expect("EURUSD");
-                let mut count = 0u64;
-                // Read first 10 chunks only (for speed)
-                for _ in 0..10 {
-                    if let Some(chunk) = iter.next_chunk() {
-                        count += chunk.len() as u64;
-                    } else {
-                        break;
-                    }
-                }
-                count
-            })
-        })
-        .collect();
-
-    let counts: Vec<u64> = handles.into_iter().map(|h| h.join().unwrap()).collect();
-
-    // All threads should read the same number of records
-    assert!(counts[0] > 0);
-    assert!(
-        counts.iter().all(|&c| c == counts[0]),
-        "All threads should read the same amount: {counts:?}"
-    );
-    println!("Concurrent access: 4 threads each read {} records", counts[0]);
-}