Compile the modules once, and clear the warning backlog

`cargo check --all-targets` emitted 32 warnings; it now emits none. Almost all
of them came from one cause: main.rs declared `mod config/fleet/printer/uplink`
while lib.rs declared the same modules, so every file was compiled twice, once
per crate. In the binary copy nothing is externally reachable, so every
`pub use` re-export and every method that only an example or a test calls was
genuinely dead - and the warnings were real about the binary while being wrong
about the library's actual public API. main.rs now uses the library crate and
keeps only `mod go2rtc`, which nothing else needs.

The rest, individually:

- `ModuleInfo` was re-exported from bambu_commands but not from printer, so it
  was unreachable even though `GetVersionResponse.module` is a public field of
  type `Vec<ModuleInfo>` - a caller could not name the type of a field it can
  read. Re-exported rather than dropped.
- Three stub `publish` impls take a payload they do not use yet; renamed to
  `_payload`.
- One unused import in examples/test_bambu_certs.rs.

That leaves the `async_fn_in_trait` lint on GenericPrinter, which is now
`allow`ed with the reasoning written down rather than left to fire on every
build. The old comment said it "only matters once this is wired into the
multi-threaded daemon", which has since happened - so the question got
answered instead of deferred: the futures cannot be required to be `Send`, and
that is fine because every printer is reached through one Arc<Mutex<Vec<..>>>
from the uplink's own task, so they are created and polled in the same place.
The note says what to do if a printer ever gets its own task.

No behaviour change. Tests still pass and the daemon was re-verified against
the live P1P and H2C through the full backend path.

examples/test_bambu_certs.rs

@@ -7,7 +7,7 @@
 //! this trust question.
 
 use continuum_proxy::fleet;
-use continuum_proxy::printer::{GenericPrinter, PrinterHandle};
+use continuum_proxy::printer::PrinterHandle;
 
 fn main() -> anyhow::Result<()> {
     let printers = fleet::load(std::path::Path::new("printers.toml"))?;