Add printers.toml (gitignored) + real TLS cert test against actual hardware

New:
- printers.example.toml (committed template) / printers.toml (gitignored,
  real hosts + access codes don't belong in git) — a list of real printers
  with vendor, host, access_code/api_key/com_port, and an optional
  per-printer CA override.
- src/fleet.rs loads that file into ready-to-use PrinterHandles, matching
  vendor strings to the right constructor.
- BambuPrinter::test_tls_handshake() does a real (blocking, one-shot) TLS
  handshake to port 8883 using that printer's configured BambuTls trust
  mode — no MQTT protocol, just 'does the certificate verify'. Added
  native-tls and toml as direct dependencies for this.
- examples/test_bambu_certs.rs loads printers.toml and runs the handshake
  test against every Bambu entry.

Also fixes a real bug found while testing against unreachable IPs: plain
TcpStream::connect has no timeout and hung indefinitely on an offline
printer — switched to connect_timeout (5s).

Verified with cargo check --all-targets (0 errors) and by actually running
test_bambu_certs against a local printers.toml (correctly errored on a
missing cert file, then correctly timed out against unreachable test IPs
instead of hanging).

README.md

@@ -19,6 +19,9 @@ inheritance) on something simple before adding real networking on top.
 cp .env.example .env
 cargo run                                  # the daemon: cloud uplink + go2rtc watchdog
 cargo run --example printer_polymorphism   # standalone demo, no network/env needed
+
+cp printers.example.toml printers.toml     # fill in your real printers (gitignored)
+cargo run --example test_bambu_certs       # real TLS handshake test against each Bambu printer
 ```
 
 ## Structure
@@ -27,16 +30,26 @@ cargo run --example printer_polymorphism   # standalone demo, no network/env nee
 src/
   main.rs      Runs the uplink and the go2rtc watchdog side by side
   config.rs    Loads settings from environment variables
+  fleet.rs     Loads printers.toml into ready-to-use PrinterHandles
   uplink/      WebSocket client to continuum-backend: connect, heartbeat, reconnect on drop
-  printer/     Printer trait + PrinterBase + PrinterHandle enum + one stub per vendor
+  printer/     GenericPrinter trait + PrinterBase + PrinterHandle enum + one stub per vendor
   go2rtc.rs    Restarts the go2rtc camera-restreaming process if it dies
 examples/
-  printer_polymorphism.rs   Runs all three printer stubs through one `connect()` call site
+  printer_polymorphism.rs   Runs all five printer stubs through one `connect()` call site
+  test_bambu_certs.rs       Loads printers.toml, does a real TLS handshake to each Bambu printer
 ```
 
 `src/printer/` is where the "inheritance" question lives — see that
 module's doc comment for the trait+enum pattern this project uses instead
-of class inheritance, and run the example above to see it work.
+of class inheritance, and run `printer_polymorphism` to see it work.
+
+`printers.toml` (gitignored — copy from `printers.example.toml`) holds real
+per-printer connection details: host, access code, and — since not every
+Bambu printer trusts the same certificate (see `certs/README.md`) — an
+optional per-printer CA override. `src/fleet.rs` loads it; nothing in
+`main.rs` uses it yet, but `test_bambu_certs` does, as a real (if narrow —
+just the TLS handshake, no MQTT) way to check a printer's certificate
+without needing the full MQTT client built yet.
 
 ## What's not here yet (on purpose)