Documented typed Starlark for build and plugin definitions and a two-mode plugin model

The design and roadmap documentation now reflect the project’s language and plugin strategy. Build files and plugins are written in typed Starlark (Buck2 dialect via starlark-rust) and evaluated in-process, while impure work is handled by named Rust handlers in the substrate. The docs also introduce a two-mode plugin model: declared mode (rust_binary, rust_library, rust_test in .fabrik files) gives full graph control and self-hosting, while adopted mode (rust_workspace) reads an existing Cargo.toml for drop-in adoption. The WASM plugin host phase has been removed from the roadmap.