Add print.rs (AmsState/AmsUnit/AmsTray/XcamSettings), fix print vs pushing

Mirrors continuum-schemas' schemas/bambulab/ restructuring: PushallResponse
removed (it never existed on the wire — pushall's request goes to
"pushing", but the state-push stream it triggers arrives under "print",
a different root key, not a direct reply). BambuReport::Pushing(...) ->
BambuReport::Print(PrintReport).

AmsTray is an untagged enum (Empty/Loaded) rather than one struct full of
Option<T> fields, matching the JSON Schema's oneOf for the same slot -
#[serde(deny_unknown_fields)] on EmptyTray is load-bearing: untagged enums
try variants in order, and a loaded tray (superset of Empty's one field)
would otherwise incorrectly match Empty first.

Verified against the full real ams/xcam example from the conversation via
examples/bambu_commands_demo.rs - both the empty and loaded tray in that
payload deserialize to the correct enum variant.

src/printer/mod.rs

@@ -35,7 +35,8 @@ mod prusa;
 
 pub use bambu::{BambuGenericPrinter, BambuTls, BambuV1Printer, BambuV2Printer};
 pub use bambu_commands::{
-    BambuReport, BambuRequest, CommandEnvelope, GetVersionRequest, GetVersionResponse, PushallRequest, PushallResponse,
+    AmsState, AmsTray, AmsUnit, BambuReport, BambuRequest, CommandEnvelope, EmptyTray, GetVersionRequest,
+    GetVersionResponse, LoadedTray, PrintReport, PushallRequest, XcamSettings,
 };
 pub use klipper::KlipperPrinter;
 pub use prusa::{PrusaLinkPrinter, PrusaSerialPrinter};