Function mark_optional_packages 

pub fn mark_optional_packages(graph: &mut LockfileGraph)

Set each package’s optional flag the way pnpm marks the snapshots: section: a package is optional: true when it is reachable only through optional dependency edges (the classic case is every @esbuild/* platform native sitting under esbuild’s optionalDependencies). pnpm derives this during resolution; aube recomputes it as a post-resolve pass so freshly resolved lockfiles carry the same markers pnpm writes instead of an empty {} snapshot.

Algorithm: seed a required set from every non-optional direct dependency of every importer, then walk each required package’s non-optional edges. A package’s non-optional edges are its dependencies minus its optional_dependencies, because the pnpm parser mirrors active optional edges into dependencies. Any package not reached this way is optional. A single fully-required path keeps a package required even when other paths to it are optional, matching pnpm.