Once narrows its focus to cacheable scripts and the runtime cache

The project has been renamed from Fabrik to Once and its scope narrowed to cacheable scripts and the runtime cache. Language-specific build targets (Apple, Elixir, Go, Rust) have been removed, the CLI and crates are renamed, and the documentation is updated to reflect the new focus. If you were using the previous build-system features, you will need to switch to the script-oriented workflow.