benches: baseline results

Two compile fixes:
- tokio_favored.rs bench_mpsc_smarm: consumer spawn closure returned u64 via
  bare 'count' tail expression; smarm::Runtime::run() requires FnOnce()->().
  Fixed to 'let _ = count;'. Same fix on the consumer.join() call site.
- smarm_favored.rs bench_unc_smarm: same pattern, same fix.

Baseline run: Intel Xeon @ 2.80GHz, 1 core, kernel 6.18.5, rustc 1.95.0,
smarm 0.3.0, no RUSTFLAGS. Single-CPU sandbox — N-thread rows identical to
1-thread; scaling sweep limited to 1 thread.

Notable findings:
- deep_recursion: tokio wins (22 vs 62 us); mmap stack alloc cost dominates
  for single-use actors at depth 500.
- yield_in_hot_loop: tokio wins (138 vs 182 ms); smarm mutex overhead on
  yield_now exceeds expected naked-switch advantage on 1 CPU.
- mpsc_contention/uncontended_channel/catch_unwind_panics: smarm wins as
  predicted.
- spawn_storm_busy: smarm 47x slower; global mutex saturated by bg yielders.

benches/baseline-output/multi_scheduler.txt

@@ -0,0 +1,34 @@
+smarm multi-scheduler benchmarks
+available parallelism: 1 threads
+PRIME_N=400000, WORKERS=64, PING_ROUNDS=10000, SPAWN_COUNT=1000
+
+================================================================================
+  Fan-out/fan-in: count primes in [2, 400000) across 64 workers
+================================================================================
+               runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+     baseline (serial) |        33860 |      18581 |      18519 |      18905
+   smarm single-thread |        33860 |      19467 |      19354 |      22082
+        smarm 1-thread |        33860 |      19345 |      19287 |      19653
+  tokio current_thread |        33860 |      18681 |      18591 |      18982
+    tokio multi-thread |        33860 |      18948 |      18726 |      19212
+
+================================================================================
+  Ping-pong: 10000 round-trips between two actors
+================================================================================
+               runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+   smarm single-thread |        10000 |       2547 |       2473 |       2841
+        smarm 1-thread |        10000 |       2546 |       2518 |       2702
+  tokio current_thread |        10000 |       1221 |       1168 |       1366
+    tokio multi-thread |        10000 |       1487 |       1316 |       2331
+
+================================================================================
+  Spawn throughput: 1000 actors spawned and joined
+================================================================================
+               runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+   smarm single-thread |         1000 |       8934 |       8066 |      12204
+        smarm 1-thread |         1000 |       8102 |       8041 |      10849
+  tokio current_thread |         1000 |        212 |        210 |        331
+    tokio multi-thread |         1000 |        330 |        301 |        604