GUI: draw the forecast line solid instead of dashed

Same color, same alpha-based fade distinguishing it from the actual
measured trace - just '-b' instead of '--b'. Updates the surrounding
docstrings/comments and README that described it as dashed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DkkuG48uHFCGKe6dPSERFk

README.md

@@ -293,9 +293,9 @@ disabled, its temperature/heat-rate setpoint controls are inactive.
 ### Forecast vs. actual duration
 
 The GUI's Automatic tab plots two things against the same time axis: a
-*forecast* (dashed) and, once a run starts, the *actual* measured trace
-(solid) overlaid on top of it - a comparison between predicted and real
-control behavior, not just a progress display.
+*forecast* (faded, via alpha) and, once a run starts, the *actual*
+measured trace (solid, full opacity) overlaid on top of it - a comparison
+between predicted and real control behavior, not just a progress display.
 
 The forecast is computed server-side (`components/sud_forecast.py`'s
 `SudForecastEstimator`) by actually running the schedule through a
@@ -335,8 +335,8 @@ tripped - the controller stays enabled and actively holding throughout
 `WAIT_USER`, so the setpoint is what it was actually converging toward
 during the wait, not a possibly-still-mid-ramp snapshot. The corrected
 forecast is sent in full each time, so the GUI's `SudForecastPlot.
-show_forecast()` simply redraws
-the dashed line outright rather than patching it up itself.
+show_forecast()` simply redraws the (faded) forecast line outright rather
+than patching it up itself.
 
 Comparing the two lines: real-world divergence from the forecast - more
 than a transient blip during a ramp's settling - is worth investigating as