Configurable attributes via select() in TOML manifests

You can now write conditional attribute values directly in once.toml using a select map. Configuration keys match the target’s platform, architecture, SDK variant, or Mac Catalyst token, and composite keys like ios:simulator take precedence over broader keys. For example, sdk_frameworks = { select = { ios = ["UIKit"], macos = ["AppKit"] } } resolves to the right value at analysis time.