chore: scaffold empty cargo lib crate and add design spec

Initial state from `cargo new --lib` plus the technical design spec at
docs/heuropt_tech_design_spec.md, which is the source of truth for the
crate's public API and v1 acceptance criteria.

src/lib.rs

@@ -0,0 +1,14 @@
+pub fn add(left: u64, right: u64) -> u64 {
+    left + right
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+}