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/primes.txt

@@ -0,0 +1,7 @@
+Counting primes in [2, 200000) across 16 workers, 5 iterations each
+
+     runtime |    primes found |           median |             min |             max
+--------------------------------------------------------------------------------
+    baseline | primes:  17984 | median:     7244 µs | min:     7231 µs | max:     7509 µs
+       smarm | primes:  17984 | median:     7592 µs | min:     7505 µs | max:     8130 µs
+       tokio | primes:  17984 | median:     7263 µs | min:     7225 µs | max:     9067 µs