Configure attributes per platform with select() in manifests

TOML manifests now support a select shape for attributes that vary by configuration, such as sdk_frameworks = { select = { ios = ["UIKit"], macos = ["AppKit"] } }. Branches match platform, sdk_variant, arch, mac_catalyst, or composite keys like ios:simulator, with the most specific key winning. The graph loader rejects select on attributes that the target kind marks as non-configurable or on configuration-input attributes like platform.