Simplify boilerplate for learning: drop real protocol code, minimal main.rs

Removed adapters/ (rumqttc MQTT, suppaftp FTPS, reqwest HTTP), plate_changer/
(tokio-serial), discovery/, and cache.rs (rusqlite) — src/printer/ already
covers 'talk to a printer' as simple stubs, so keeping a second, more complex
version alongside it was redundant. main.rs goes from 6 concurrent tasks to
2 (uplink + go2rtc). uplink/ drops channels, jitter, and Send-bound futures.

Verified with cargo build + cargo run --example printer_polymorphism.

src/uplink/protocol.rs

@@ -1,42 +1,17 @@
 use serde::{Deserialize, Serialize};
 
-/// Messages sent from this gateway up to `continuum-backend`'s `/ws/edge/v1` route.
+/// What this gateway sends up to `continuum-backend`'s `/ws/edge/v1` route.
 #[derive(Debug, Clone, Serialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ClientMessage {
-    Hello {
-        gateway_id: String,
-        farm_id: String,
-        token: String,
-        version: &'static str,
-    },
-    Heartbeat {
-        seq: u64,
-    },
-    Telemetry {
-        printer_id: String,
-        payload: serde_json::Value,
-    },
-    TaskResult {
-        task_id: String,
-        ok: bool,
-        error: Option<String>,
-    },
+    Hello { gateway_id: String, farm_id: String, token: String },
+    Heartbeat { seq: u64 },
 }
 
-/// Messages received from `continuum-backend` over the same connection.
+/// What `continuum-backend` sends back over the same connection.
 #[derive(Debug, Clone, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ServerMessage {
-    HelloAck {
-        session_id: String,
-    },
-    HeartbeatAck {
-        seq: u64,
-    },
-    Task {
-        task_id: String,
-        kind: String,
-        payload: serde_json::Value,
-    },
+    HelloAck { session_id: String },
+    HeartbeatAck { seq: u64 },
 }