CI wall-time gate

Run the speed benchmark in CI and fail the build when a path is slower than the native baseline by some threshold.

Why not: Wall-clock numbers on shared CI runners are noisy — neighbour load, throttling and cold caches swing a sub-second spawn by more than the regression threshold would be, so a run can cross the bar on measurement noise alone. The signal-to-noise ratio does not support a pass/fail gate.

The need is already met elsewhere: speed regressions are caught by the committed colocated *.bench.md artifacts diffed offline (benchmarking), with the 🏎️ Bench CI job running plain vitest bench as an executes-clean smoke signal; correctness regressions hard-fail the CI coverage shards via the plain-Vitest differential suite (correctness).

Rejected

Ideas virrun decided against — one page per idea with the rationale, so nothing is re-argued.

Hardlink/CAS flush

Speed up write-back by hardlinking or content-addressing the flush instead of byte-copying produced files.