Function arena_size_for 

pub const fn arena_size_for(cbs: usize) -> usize

phase-271 — default arena bytes for a per-entry executor holding cbs callback slots. Scales the build-time ARENA_SIZE (which is sized for the default MAX_CBS) linearly by cbs, so a per-entry ExecutorSizing with a caller-chosen cbs gets the same per-slot arena budget the global default used — a fat entry (more callbacks) gets a larger arena, a lean one a smaller one — without a workspace-global NROS_EXECUTOR_ARENA_SIZE. Floored at the full default so no per-entry executor is ever smaller than a single-slot build would have been (the arena is a worst-case upper bound; over-provision is safe, under-provision fails entity creation).