Revert Sud hold "duration" back to minutes

Reverts 19dc7a9 ("Fix Sud hold duration units: seconds, not minutes")
per explicit instruction - duration is minutes again. Re-adds the
*60.0 conversion in Sud._advance() (hold_remaining is ticked in
simulated seconds), the matching conversion in the forecast plot's
_estimate_course(), and flips the README wording back.

README.md

@@ -134,7 +134,7 @@ Files under `sude/` describe a brew's mash schedule ("Sud"): `pot_mass`,
 - a ramp — `"ramp": {"rate": ..., "temp": ...}` — ramp to `temp` at `rate`
   (°C/min), or
 - a hold — `"hold": {"duration": ...}` — hold the current target for
-  `duration` seconds (omit/`0` for an immediate step).
+  `duration` minutes (omit/`0` for an immediate step).
 
 Both `ramp` and `hold` carry a `stirrer` block (`speed`, `interval_time`,
 `on_ratio`): `interval_time: 0` runs the stirrer continuously at `speed`;