Terrane test timing scoreboard

Bounded-parallel wall-clock measurements from stable Rust libtest plus compiler-owned nested case timings. Libtest durations are inferred from its deterministic alphabetical queue and completion events. Dependency-free generated-crate compilation has its own shared timing row instead of being estimated per case. A successful workspace run prunes rows not observed in that run; partial or failed runs preserve them. Durations include small runner and output overhead and are intended for relative development feedback.