chore: add Rust project scaffold

Add the initial Cargo package, formatter configuration, ignore rules, lockfile,
and placeholder binary entry point. This establishes the repository baseline so
future solver work can be reviewed as focused behavior changes.

The scaffold intentionally contains no Sudoku behavior yet. It only captures the
starting point of the project as generated and configured locally.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests

Refs: none

Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "sudoku-ai"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+
+[lints.rust]
+unsafe_code = "forbid"
+
+[lints.clippy]
+pedantic = { level = "warn", priority = -1 }
+todo = "warn"
+unwrap_used = "warn"
+
+[profile.release]
+lto = true
+strip = true
+codegen-units = 1
+debug = false
+panic = "unwind"