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.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/printer/mod.rs

@@ -38,7 +38,7 @@ mod prusa;
 pub use bambu::{BambuPrinter, BambuProtocolPreference, BambuProtocolVersion, BambuTls};
 pub use bambu_commands::{
     AmsState, AmsTray, AmsUnit, BambuReport, BambuRequest, CommandEnvelope, EmptyTray, GetVersionRequest,
-    GetVersionResponse, LoadedTray, PrintReport, PushallRequest, XcamSettings,
+    GetVersionResponse, LoadedTray, ModuleInfo, PrintReport, PushallRequest, XcamSettings,
 };
 pub use klipper::KlipperPrinter;
 pub use prusa::{PrusaLinkPrinter, PrusaSerialPrinter};
@@ -59,6 +59,19 @@ pub struct PrinterBase {
 
 /// The shared interface - think `interface GenericPrinter` (Java/TS) or an
 /// ABC (Python). Every vendor's struct implements this.
+///
+/// The `allow` is a considered decision, not a brush-off. `async fn` in a
+/// public trait means callers cannot require the returned futures to be
+/// `Send`, so such a future cannot be moved to another OS thread. That is
+/// fine here, and checked: the daemon does run on tokio's multi-threaded
+/// runtime, but every printer is reached through one `Arc<Mutex<Vec<..>>>`
+/// from the uplink's own task (see `uplink::SharedFleet`), so these futures
+/// are created and polled in the same place. Revisit if a printer ever gets
+/// its own task - the fix is to desugar to
+/// `fn connect(&mut self) -> impl Future<Output = ..> + Send`, which cannot
+/// be done without changing every implementation, which is why it is not
+/// done pre-emptively.
+#[allow(async_fn_in_trait)]
 pub trait GenericPrinter {
     /// Read-only access to the shared fields.
     fn base(&self) -> &PrinterBase;
@@ -67,14 +80,11 @@ pub trait GenericPrinter {
     /// so there's nothing sensible to share here. Each `impl` in
     /// `bambu.rs`/`prusa.rs`/`klipper.rs` provides a totally different
     /// body - that's the override.
-    ///
-    /// `cargo check` will warn about `async fn` in a public trait here.
-    /// That's about a more advanced concern (whether the resulting work can
-    /// safely move across OS threads) that only matters once this is wired
-    /// into the multi-threaded daemon - safe to ignore for now.
     async fn connect(&mut self) -> Result<(), PrinterError>;
 
-
+    /// Sends one already-encoded message by whatever transport this vendor
+    /// uses. Lower level than the methods below and mostly a building block
+    /// for them - `bambu.rs` publishes MQTT with it; the stubs only log.
     async fn publish(&mut self, payload: String) -> Result<(), PrinterError>;
 
     /// REQUIRED, no default body: `percent` is 0-100, but how it actually