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).

.gitignore

@@ -6,3 +6,4 @@ Cargo.lock
 *.sqlite3
 .env
 *.log
+printers.toml