Replace proto-based bambu_protocol with plain-Rust bambu_commands/

One file per command (get_version.rs, pushall.rs), each holding its
request AND response together. CommandEnvelope (sequence_id + command) is
the shared-fields piece every command embeds via #[serde(flatten)] — the
'inheritance' replacement for these structs, same composition idea as
BambuGenericPrinter in bambu.rs, just expressed as a struct field instead
of impl-block delegation.

examples/bambu_commands_demo.rs round-trips both commands against Bambu's
exact real JSON (from the original conversation) and confirms byte-for-
byte matches, same as the removed proto version did — this replacement is
behaviorally identical, just without protoc/build.rs in the loop.

src/printer/mod.rs

@@ -29,12 +29,14 @@
 //! `tokio::spawn`, not yet).
 
 mod bambu;
-mod bambu_protocol;
+mod bambu_commands;
 mod klipper;
 mod prusa;
 
 pub use bambu::{BambuGenericPrinter, BambuTls, BambuV1Printer, BambuV2Printer};
-pub use bambu_protocol::{BambuReport, BambuRequest};
+pub use bambu_commands::{
+    BambuReport, BambuRequest, CommandEnvelope, GetVersionRequest, GetVersionResponse, PushallRequest, PushallResponse,
+};
 pub use klipper::KlipperPrinter;
 pub use prusa::{PrusaLinkPrinter, PrusaSerialPrinter};