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.

.gitignore

@@ -1,4 +1,6 @@
 public
+target
 
 # Local Netlify folder
 .netlify
+target/