Fix Sud hold duration units: seconds, not minutes

_advance() multiplied "duration" by 60, treating it as minutes;
stirrer_speed (percent) and stirrer_time_on/off (seconds) were already
handled correctly with no conversion. Also fix the README's "duration
minutes" wording to match.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -132,7 +132,7 @@ Files under `sude/` describe a brew's mash schedule ("Sud"): pot weight,
 malt/water weights, and a flat `schedule` list of steps, each either:
 
 - `"type": "heat"` — ramp to `temp` at `rate` (°C/min), or
-- `"type": "hold"` — hold the current target for `duration` minutes (omit
+- `"type": "hold"` — hold the current target for `duration` seconds (omit
   for an immediate, zero-duration step).
 
 Every step also carries its own stirrer timing (`stirrer_speed`,