fix CI: skip concurrent/stress tests under Miri (too slow)

Miri runs tests 100-1000x slower than native. Thread spawning causes
extreme timeouts. These tests run fine on native (Test job passes).

Author: bravo1goingdark

tests/concurrent.rs

@@ -1,3 +1,5 @@
+#![cfg(not(miri))] // Thread spawning is extremely slow under Miri
+
 use std::sync::Arc;
 use std::thread;
 

tests/stress.rs

@@ -1,3 +1,5 @@
+#![cfg(not(miri))] // Thread spawning is extremely slow under Miri
+
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 use std::thread;