Function platform_of 

pub fn platform_of(unit: &str) -> Option<Platform>

v0.19 decision 0024: which first-party units are platform-native — consuming one locks its deployment unit to the returned platform. This is the metadata that drives effective-platform computation, env resource typing, and wrangler.toml stanza derivation; no user-facing marker syntax exists (additive later, when third-party platform adapters become a goal).