Move Sud step's target temperature from ramp.temp to step.temperature

A step's target temperature applies to the whole step (it's what a
ramp ramps to and what a hold then holds at), not just its ramp phase
- promote it from a nested "ramp": {"temp": ...} to a step-level
"temperature" field, defaulted like descr/grain_mass/etc. Updates all
consumers (Sud._build_step, SudTask, the GUI's forecast estimator,
demo_sud.py) and migrates sude/sud_0010.json and the README.

README.md

@@ -130,17 +130,19 @@ environment (`$WORKON_HOME`/`$BREWPI_HOME`) on a Raspberry Pi-style deployment.
 
 Files under `sude/` describe a brew's mash schedule ("Sud"): `pot_mass`,
 `pot_material` (fixed for the whole brew), and a `steps` list, each a ramp, a
-hold, or both:
+hold, or both, plus a step-level `temperature` (the target a ramp step ramps
+to, and what a hold step holds at):
 
-- a ramp — `"ramp": {"rate": ..., "temp": ...}` — ramp to `temp` at `rate`
+- a ramp — `"ramp": {"rate": ...}` — ramp to `temperature` at `rate`
   (°C/min), and/or
 - a hold — `"hold": {"duration": ...}` — hold the current target for
   `duration` minutes (omit/`0` for an immediate step).
 
-A step with both ramps to `temp` and then holds there for `duration` -
-useful for a mash rest ("ramp to 63°C, then hold 40 min") without needing two
-separate schedule entries. `user_wait_for_continue`/`user_message` (below)
-apply once the whole step is done, i.e. after the hold phase if there is one.
+A step with both ramps to `temperature` and then holds there for `duration`
+- useful for a mash rest ("ramp to 63°C, then hold 40 min") without needing
+two separate schedule entries. `user_wait_for_continue`/`user_message`
+(below) apply once the whole step is done, i.e. after the hold phase if
+there is one.
 
 Both `ramp` and `hold` carry a `stirrer` block (`speed`, `interval_time`,
 `on_ratio`): `interval_time: 0` runs the stirrer continuously at `speed`;