refactor: migrate project from C++ to Rust

- establish basic Rust project layout.
- move all old code into independent directory.

Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+resolver = "3"
+members = ["wfassoc", "wfassoc_dylib", "wfassoc_exec"]
+
+[workspace.dependencies]
+thiserror = "2.0.12"