Faster native project discovery

Once now discovers native Cargo, Mix, and future ecosystem projects with one shared filesystem pass. Workspace include and exclude patterns prune unrelated directories before Once enters them, reducing startup work in large repositories without changing the targets that discovery produces.

Discovery also keeps its own memory usage predictable. Project kinds that own nested packages retain only their shallowest matching roots, while kinds that recognize every nested project have a 16 mebibyte retained-result limit. Once stops with guidance for narrowing the workspace if that limit is reached, instead of allowing the number of matching directories to cause unbounded memory growth.