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.

Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "nrd-sh"
+version = "0.1.0"
+edition = "2024"
+description = "Bespoke static site compiler for nrd.sh"
+license = "MIT"
+
+[dependencies]
+pulldown-cmark = "0.12"
+gray_matter = "0.2"
+maud = "0.26"