feat: initialize rust project scaffold

Add minimal Rust project structure for bespoke site compiler:

- rust-toolchain.toml: stable channel with rustfmt, clippy, rust-src
- Cargo.toml: nrd-sh v0.1.0 with pulldown-cmark, gray_matter, maud
- src/main.rs: stub entrypoint

Verified with cargo check.

src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("nrd.sh compiler v{}", env!("CARGO_PKG_VERSION"));
+}