docs: refresh Forecast vs. actual duration for the real-temperature anchoring fixes

Documents the recent SudTask.send_forecast()/recv() changes (anchored at
get_theta_ist_set(), re-anchored on every fresh Start) and the two
spurious-divergence fixes: client/brewpi_gui.py's forecast_history t=0
seeding, and the hardcoded last_theta_ist=20 bug in both TempController
variants.

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

README.md

@@ -308,11 +308,20 @@ server uses - plant params (`M`/`C`/`L`/`Td`) aren't a constructor
 argument at all, since they come from the doc being forecast itself
 (`Sud.derive_plant_params()`, same as the real run), not from any generic
 default. A multi-hour brew simulates in well under a second since it's
-pure CPU-bound iteration, no real time/IO involved. This
-is deliberately *not* recomputed as a run progresses: a forecast that keeps
-re-anchoring itself to match whatever's actually happening would no longer
-be a meaningful baseline to compare reality against. It's computed once,
-up front, and left alone.
+pure CPU-bound iteration, no real time/IO involved.
+
+It's anchored to the real current temperature
+(`TempControllerBase.get_theta_ist_set()` - the raw sensor reading, valid
+even before this controller's first `process()` tick, unlike
+`get_theta_ist()`), not a cold start at ambient - `SudTask.send_forecast()`
+computes it once on Load (a preview of what's coming), and again on every
+fresh `Start` (not a Pause→resume, which keeps whatever forecast a run
+already established rather than discarding it mid-run), since the real
+temperature can have drifted in the meantime
+(time passing, manual heating via the Manual tab in between). Once a run
+is actually progressing, though, it's deliberately *not* continuously
+re-anchored to match whatever's happening: a forecast that keeps chasing
+reality would no longer be a meaningful baseline to compare it against.
 
 `SudTask._send_forecast()` thins the curve to at most `MAX_FORECAST_POINTS`
 (1000) before broadcasting it - a fine enough `dt` over a multi-hour brew
@@ -366,6 +375,33 @@ speedup runs measurably below it (confirmed: ~80x instead of a configured
 100x), which on its own used to produce a divergence large enough to look
 like a control problem.
 
+Two other sources of spurious (non-control-related) divergence, both
+fixed:
+
+- `client/brewpi_gui.py`'s actual-trace history used to only get its first
+  sample at `on_plot_timer()`'s next 1-second (real time) tick - under a
+  real `sim_warp_factor`, simulated time has already moved measurably past
+  `t=0` by then, leaving a small visible gap between the forecast's exact
+  `t=0` anchor and where the actual line first appears. `on_sud_changed()`
+  now seeds `forecast_history` with the real `(elapsed, plot_temp_ist)`
+  point the moment the first `Elapsed` push of a *fresh* run arrives
+  (gated on elapsed still being small, `FRESH_RUN_ELAPSED_THRESHOLD_S`, so
+  reconnecting mid-brew doesn't get a bogus point plotted at `x=0`).
+- Both `TempController` variants used to hardcode `last_theta_ist = 20` at
+  construction, used to compute the very first `process()` tick's
+  `heatrate_ist` - a wildly bogus rate spike whenever the real starting
+  temperature wasn't actually 20 (invisible until a forecast actually got
+  anchored to something other than 20, per the fix above), which then took
+  several exponentially-smoothed ticks to decay out of the rate-PID loop,
+  visibly distorting the trajectory during exactly that window. It now
+  starts `None` and treats the first tick's rate as `0` (no real history
+  yet) instead of computing it against a hardcoded, usually-wrong
+  baseline - `SudForecastEstimator` is the case that actually surfaced
+  this: it builds a fresh `TempController` on every call, so its first
+  tick always hit this fresh, unlike the real controller, which only ever
+  goes through it once near server startup and has long since
+  self-corrected by the time any forecast comparison matters.
+
 ## Logging & analysis
 
 `tasks/sud_log.py`'s `SudLogTask` records every Sud run's measured data -