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/discovery/mod.rs

@@ -1,44 +0,0 @@
-use std::time::Duration;
-
-use serde::{Deserialize, Serialize};
-use tracing::{debug, info};
-
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub enum PrinterVendor {
-    Bambu,
-    Prusa,
-    Klipper,
-}
-
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct DiscoveredPrinter {
-    pub vendor: PrinterVendor,
-    pub host: String,
-    pub serial: Option<String>,
-}
-
-/// Periodically sweeps the LAN for printers (SSDP for Bambu, mDNS for
-/// PrusaLink/Moonraker) and reports newly-seen devices upstream.
-///
-/// This is intentionally a stub: production discovery would bind a UDP
-/// multicast socket per protocol. It's structured so each protocol's probe
-/// can be dropped in independently without touching the polling loop.
-pub async fn run(interval: Duration, tx: tokio::sync::mpsc::Sender<DiscoveredPrinter>) {
-    let mut ticker = tokio::time::interval(interval);
-    loop {
-        ticker.tick().await;
-        debug!("running LAN discovery sweep");
-
-        for printer in sweep().await {
-            info!(host = %printer.host, ?printer.vendor, "discovered printer");
-            if tx.send(printer).await.is_err() {
-                return;
-            }
-        }
-    }
-}
-
-async fn sweep() -> Vec<DiscoveredPrinter> {
-    // TODO: SSDP probe (Bambu), mDNS `_prusalink._tcp` / `_moonraker._tcp` probes.
-    Vec::new()
-}