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.

.env.example

@@ -5,25 +5,9 @@ CONTINUUM_GATEWAY_TOKEN=replace-with-provisioning-token
 
 # Cloud uplink
 CONTINUUM_UPLINK_URL=wss://api.continuum.local/ws/edge/v1
-CONTINUUM_API_BASE=https://api.continuum.local
-
-# Local state
-CONTINUUM_SQLITE_PATH=./data/edge-cache.db
 
 # go2rtc process management
 CONTINUUM_GO2RTC_BIN=/usr/local/bin/go2rtc
 CONTINUUM_GO2RTC_CONFIG=./go2rtc.yaml
-CONTINUUM_GO2RTC_API_PORT=1984
-
-# Bambu Lab printers (LAN mode) — comma-separated "serial:ip:access_code" triples,
-# normally discovered via SSDP but overridable for static configs.
-CONTINUUM_BAMBU_STATIC_PRINTERS=
-
-# PrusaLink / Moonraker discovery
-CONTINUUM_DISCOVERY_INTERVAL_SECS=30
-
-# plate changer hardware
-CONTINUUM_PLATE_CHANGER_SERIAL_PORT=/dev/ttyUSB0
-CONTINUUM_PLATE_CHANGER_BAUD=115200
 
 RUST_LOG=info,continuum_proxy=debug