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/smarm_favored.txt

@@ -0,0 +1,40 @@
+smarm smarm-favored benchmarks
+available parallelism: 1 threads
+ITERS=15 (+1 warmup, discarded)
+RECURSE_DEPTH=500, HOT_YIELDS=500000×2, UNCONT_MSGS=1000000, PANIC_TASKS=10000
+
+================================================================================
+  deep_recursion: depth 500
+================================================================================
+                   runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+            smarm 1-thread |            1 |         62 |         59 |        682
+            smarm 1-thread |            1 |         71 |         61 |        210
+      tokio current_thread |            1 |         22 |         22 |         23
+        tokio multi-thread |            1 |         44 |         38 |         79
+
+================================================================================
+  yield_in_hot_loop: 2 actors × 500000 yields (single thread)
+================================================================================
+                   runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+            smarm 1-thread |      1000000 |     182177 |     180380 |     184410
+      tokio current_thread |      1000000 |     138335 |     136097 |     141196
+
+================================================================================
+  uncontended_channel: 1→1, 1000000 msgs (single thread)
+================================================================================
+                   runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+            smarm 1-thread |      1000000 |      31473 |      28719 |      33113
+      tokio current_thread |      1000000 |      51925 |      51205 |      53043
+
+================================================================================
+  catch_unwind_panics: 10000 tasks, 50% panic
+================================================================================
+                   runtime |       result |  median µs |     min µs |     max µs
+--------------------------------------------------------------------------------
+            smarm 1-thread |        10000 |     112306 |     109702 |     119859
+            smarm 1-thread |        10000 |     114305 |     112030 |     121326
+      tokio current_thread |        10000 |     151443 |     150949 |     153800
+        tokio multi-thread |        10000 |     161344 |     160385 |     167573