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).

examples/test_bambu_certs.rs

@@ -0,0 +1,26 @@
+//! Run with: cargo run --example test_bambu_certs
+//!
+//! Loads printers.toml (copy printers.example.toml to get started, fill in
+//! your real printers) and attempts a raw TLS handshake to each Bambu
+//! printer's MQTTS port (8883) — no MQTT protocol involved, just "does the
+//! certificate verify". Prusa/Klipper entries are skipped; they don't have
+//! this trust question.
+
+use continuum_proxy::fleet;
+use continuum_proxy::printer::{GenericPrinter, PrinterHandle};
+
+fn main() -> anyhow::Result<()> {
+    let printers = fleet::load(std::path::Path::new("printers.toml"))?;
+
+    for printer in &printers {
+        if let PrinterHandle::BambuV1(bambu) | PrinterHandle::BambuV2(bambu) = printer {
+            print!("{}: ", bambu.base().name);
+            match bambu.test_tls_handshake() {
+                Ok(()) => println!("OK — certificate verified"),
+                Err(err) => println!("FAILED — {err}"),
+            }
+        }
+    }
+
+    Ok(())
+}