ci: skip stress tests in cross-platform matrix, add timeout

The cross-platform job ran cargo test --all-targets with no skips,
causing stress tests (stress_1m, mpmc_stress, etc.) to hang on
macOS runners. Added the same skip list used by the main test job
and a 5-minute timeout.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: userFRM

.github/workflows/ci.yml

@@ -124,7 +124,13 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - run: cargo check
-      - run: cargo test --all-targets
+      - run: >
+          cargo test --all-targets --
+          --skip mpmc_stress
+          --skip stress_1m
+          --skip arbitrary_capacity_mpmc_stress
+          --skip arbitrary_capacity_stress
+        timeout-minutes: 5
 
   # ── WASM build (verify no_std / WASM compatibility) ──────────────────
   wasm: