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.

Cargo.toml

@@ -20,30 +20,15 @@ path = "examples/printer_polymorphism.rs"
 [dependencies]
 tokio = { version = "1.40", features = ["full"] }
 tokio-tungstenite = { version = "0.24", features = ["native-tls"] }
-tokio-util = { version = "0.7", features = ["codec", "compat"] }
 futures-util = "0.3"
-rumqttc = "0.24"
-reqwest = { version = "0.12", features = ["stream", "json"] }
-suppaftp = { version = "6", features = ["async-native-tls"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-rusqlite = { version = "0.32", features = ["bundled"] }
-tokio-serial = "5.4"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 anyhow = "1"
 thiserror = "1"
-uuid = { version = "1", features = ["v4", "serde"] }
-rand = "0.8"
 dotenvy = "0.15"
 
-# continuum-common's rust-core crate (error types, tracing init, G-code helpers)
-# lives in the sibling `continuum-common` repo. Once that crate is published to
-# a registry or exposed over git, add it here, e.g.:
-#   continuum-common = { git = "https://git.octoturge.com/Continuum/continuum-common.git", package = "continuum-core" }
-
-[profile.release]
-opt-level = "z"
-lto = true
-codegen-units = 1
-panic = "abort"
+# Real printer protocol clients (MQTT for Bambu, HTTP for PrusaLink/
+# Moonraker, FTPS for gcode transfer) go here once you're ready to build
+# past the stubs in src/printer/ — see that module's doc comment.