All three hardware drivers (Olimex, generic_esp32, generic_esp8266) now
probe with the first request and bail immediately on failure.
Before: offline board with 4 relays + 3 inputs = 7 requests × 3 s timeout
= up to 21 s of blocking + 7 log lines per recheck cycle.
After: exactly 1 request × 3 s timeout regardless of I/O count.
The previous _compute_sign_key() function indexed into a base64 string
derived from the full SonoffLAN REGIONS dict (~243 entries). Our partial
dict only produced a 7876-char a string but needed index 7872+, so the
function must use the full dict.
Solution: pre-compute the key once from the full dict and hardcode the
resulting 32-byte ASCII key. This is deterministic — the SonoffLAN
algorithm always produces the same output regardless of when it runs.
The sonoff_ewelink driver now loads cleanly alongside all other drivers.
