Configurable attributes via select() for Apple targets

Apple rule attributes can now use select in once.toml to choose values based on the active target configuration. The TOML shape is { select = { ios = ..., macos = ..., default = ... } } and branch keys can combine platform, sdk_variant, arch, and mac_catalyst tokens with :, such as ios:simulator. The most specific matching key wins. This lets one target declare per-platform defines, SDK frameworks, or linker options without duplicating the target.