No CI job runs or even compiles the benches

[justin13888]

The workspace has 15 bench files and 61 benches, and no CI job runs any of them. There is
a mise run bench task, but neither ci.yml nor extended.yml invokes it.

Two consequences:

• The benches can rot silently. Nothing compiles them, so an API change breaks them and no gate
  notices until someone runs the task by hand.
• They carry zero assertions, so even run they would pin nothing — yet gamut-cmm bench
  numbers were the cited evidence for filing gamut-cmm: fixed-point/integer + SIMD evaluation paths (6.6x lcms2 gap) #423. Evidence that no gate reproduces is evidence
  with a short shelf life.

Minimum fix is a compile-only check (cargo bench --no-run) in the fast lane, which is cheap and
catches the rot. Whether benches should additionally assert a regression threshold — and where
those numbers would come from on preemptible shared runners — is the harder question and the
reason this is its own issue rather than part of #435.

docs/testing.md explicitly places benches outside what it governs, so this is not blocked on the
testing corpus pass.

Refs #435