Add Printer trait + PrinterHandle enum domain model with polymorphism example

src/lib.rs

@@ -0,0 +1,6 @@
+//! Library surface for `continuum-proxy`. The daemon itself is a binary
+//! (`src/main.rs`); this crate root exists so standalone examples (see
+//! `examples/`) and future integration tests can `use continuum_proxy::...`
+//! without duplicating module declarations.
+
+pub mod printer;