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.

rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "stable"
+components = ["rustfmt", "clippy", "rust-src"]