feat(#310): show the measured rate and step time beside the set speed

[octalide]

Closes #310

Plan

• cellar.pace: a Rate measured from the shown generation against the clock, in windows of at least 0.25 s that hold a generation, each moving the reading halfway. A pause, a reset (generation going back) or a full world drops it, so no stale number is shown. A HashLife step moves the generation by 2^k, so the same measure reads in generations a second there too. A run that completes nothing for 4 s drops the reading.
• Step time: sim.step_ns, shown through world.Shown. On the GPU it is the timestamp-measured slot cost (smoothed in gpu already) times the slots a generation steps (gpu.step_ns). On the chunk engine and HashLife it is the host step's wall time, smoothed by an eighth, cleared on play/pause. A GPU with no timestamps has no step time unless uncapped, where the interval between generations is the step's wall time (pace.step_for); capped it shows a dash since the interval would only echo the set speed.
• Top readout: speed 41 of 60/s, or speed 41/s max uncapped, - of 60/s while paused. Chosen because it reuses the existing field and reads measured against set at a glance. HashLife keeps its x2^k suffix.
• Stats panel: rate (gen/s) and step rows beside speed, a dash while paused.

Decisions the issue did not settle: the window and smoothing constants, the dash while paused, and the GPU-without-timestamps fallback above.

Ran: full mach test . under a private kwin (passes), release build, mach fmt --check .. Captures of both readouts (paused, chunk GPU, CPU, HashLife) are in the worktree's caps/. An uncapped capture could not be taken without key injection.