Stats for Nerds tab, KaTeX methods, weather icons, outlook to top

New Stats for Nerds tab over a new read-only /api/nerd endpoint: Kalman NIS
and covariance per signal, both compensators, all 18 RLS heads with trace(P)
against the cap, |theta|, EWMA RMSE, conformal alpha against target, realised
coverage and ensemble weights, plus per-head feature attribution over the 33
standardised weights, the Mahalanobis and Page-Hinkley detector state,
climatology harmonics and precipitation coefficients.

Methods overhaul: KaTeX now renders the equations. They were previously passed
through .replace(/[{}\\]/g,' '), which stripped every brace and backslash and
turned real mathematics into mush. Stages 2, 3, 5, 6 and 7 gained full
derivations (RLS normal equations and the trace cap, Joseph-form Kalman with
NIS, adaptive conformal with its coverage limit, ridge harmonic regression with
anomaly decay) and a per-symbol legend rendered inline.

Conditions ahead gains weather icons chosen from measured cloud index, solar
elevation and temperature rather than the barometric class alone, so a fine
barometer under overcast draws a cloud and after sunset draws a moon. Snow is
selected on temperature.

Seven day outlook moves to the top of Live, directly under the nav. Tab renamed
Models and Calibration.

Verified in Chromium at 1600x900: Live, History, Models and Nerd all report
zero scrollbars, zero clipping, no page scroll, zero console errors. Methods
keeps its documented prose scroller. Backtest numerically unchanged.

docs/DESIGN.md

@@ -323,7 +323,7 @@ consecutive readings are the only tell.
 
 | Symptom | Knob | Direction |
 |---|---|---|
-| Temperature reads consistently high | Calibrate from the Models and calibration tab, or `sensor.cpu_heat_k` | Raise |
+| Temperature reads consistently high | Calibrate from the Models and Calibration tab, or `sensor.cpu_heat_k` | Raise |
 | Humidity reads consistently off | Calibrate against a reference hygrometer, or `sensor.hum_offset` | Either |
 | Readings over-smoothed, lag real change | `sensor.kalman_q_temp` | Raise |
 | Rates look noisy | `sensor.kalman_q_*` down, or `kalman_r_*` up | |