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.

examples/bambu_commands_demo.rs

@@ -0,0 +1,35 @@
+//! Run with: cargo run --example bambu_commands_demo
+//!
+//! Round-trips both commands against the exact JSON from Bambu's real
+//! protocol — proves the plain hand-written struct + serde approach
+//! matches byte for byte, same as the proto-based version did, with none
+//! of the build.rs ceremony.
+
+use continuum_proxy::printer::{CommandEnvelope, GetVersionRequest, GetVersionResponse, PushallRequest};
+
+fn main() {
+    // --- get_version request ---
+    let req = GetVersionRequest {
+        envelope: CommandEnvelope { sequence_id: "0".to_string(), command: "get_version".to_string() },
+    };
+    let actual = serde_json::to_string(&req).unwrap();
+    let expected = r#"{"sequence_id":"0","command":"get_version"}"#;
+    println!("get_version request:  {actual}");
+    println!("matches Bambu's shape: {}\n", actual == expected);
+
+    // --- get_version response, deserialized from Bambu's real payload ---
+    let real_response = r#"{"command":"get_version","module":[{"hw_ver":"","name":"ota","sn":"","sw_ver":"01.01.01.00"},{"hw_ver":"","name":"xm","sn":"","sw_ver":"00.00.00.00"}],"sequence_id":"0"}"#;
+    let parsed: GetVersionResponse = serde_json::from_str(real_response).unwrap();
+    println!("get_version response parsed fine, no result/reason present: {parsed:#?}\n");
+
+    // --- pushall request ---
+    let pushall = PushallRequest {
+        envelope: CommandEnvelope { sequence_id: "0".to_string(), command: "pushall".to_string() },
+        version: 1,
+        push_target: 1,
+    };
+    let actual = serde_json::to_string(&pushall).unwrap();
+    let expected = r#"{"sequence_id":"0","command":"pushall","version":1,"push_target":1}"#;
+    println!("pushall request:       {actual}");
+    println!("matches Bambu's shape: {}", actual == expected);
+}