New canonical [[target]] TOML shape and runtime metadata

Fabrik is now centered on a canonical [[target]] TOML shape that uses rule = "..." to declare built-in targets such as rust.binary, apple.simulator_app, and command. Existing [[rust.*]], [[apple.*]], and [[task]] declarations are still accepted as compatibility sugar and lower to the same internal target model. Runnable targets can now declare runtime capabilities through a nested [target.runtime] metadata block. The Apple simulator target has been renamed from [[apple.ios_app]] to [[apple.simulator_app]] with platform = "ios", with the old name kept as compatibility. Starlark is documented as the intended rule and macro authoring layer, leaving Fabrik in charge of scheduling, caching, CAS, actions, and runtime sessions.