Benchmarking: (std bench) and bjo bench, with the card game

A new section 2.9 between Testing and Performance: time-it, a benchmark file
for the deck, how a benchmark is measured, setup and parameters, the menu,
--detail and --scaling, --compare-within, saving and comparing, --against
with --processes, the flags, and --serve. Every output is from a real run.
It shows the weakness of comparing two processes too: unchanged benchmarks
called faster or SLOWER by a few percent, and how --processes 6 settles it.

The standard library chapter gets std/bench, std/stopwatch says time-it is
(std bench)'s, and modules/std/bench/ has the reference pages for (std
bench), (std bench stats) and (std bench store).

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-7)

Co-Authored-By: eca-agent <git@eca.dev>

modules/std/bench/stats.sz

@@ -0,0 +1,3 @@
+(meta (title "(std bench stats)"))
+
+@generate-docs[#:module "std bench stats"]