* perf(terminal): eliminate dense control and frame gate regressions
* test(terminal): keep gate labels in valid expect shape
* test(terminal): expose the surviving sub-threshold control-density case
The only adverse strip fixture sat at 50% control density, which is exactly
where the fallback fires and wins. A shape at 31 controls per 64-unit block
evades the trigger and still loses to the per-character legacy (0.67x), so the
benchmark structurally could not show it.
Add that fixture, pin both density literals in the staleness guard so a retune
fails loudly instead of silently measuring a boundary that moved, and export
the probe constant the equivalence test was hardcoding.
c25d85cc4c