docs: update elevation docs and changelog for two-button recalculation and DEM fix

docs/user-guide.md

@@ -73,20 +73,23 @@ Click **Edit** on any activity to:
 
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-### Recalculating elevation from terrain data
+### Recalculating elevation
 
-If an activity shows an unrealistic elevation gain (common with GPS-only devices on flat
-routes, or with older Garmin/Wahoo files), the edit drawer has a
-**"Recalculate from terrain map (DEM)"** button.
+If an activity shows an unrealistic elevation gain, the edit drawer has two buttons:
 
-Clicking it replaces the recorded GPS altitude with SRTM terrain data from the
-[Open-Elevation API](https://open-elevation.com) and recomputes the gain and loss. The
-elevation chart and the summary stats both update. This usually brings the numbers in
-line with what Strava or your device's app reports.
+**📐 Recalculate (hysteresis)** — recomputes gain and loss from the original recorded
+elevation using a noise-filtering dead-band algorithm. Fast and offline — no network
+call. Best for devices with a barometric altimeter (Garmin, Karoo, Wahoo) whose
+elevation data is accurate but was extracted before the noise-filtering was improved.
 
-> **Note:** The correction requires a GPS track (activities marked *No GPS* cannot be
-> corrected). The DEM has ~30 m horizontal resolution, so very short or indoor activities
-> are not meaningfully improved.
+**⛰ Recalculate (DEM)** — replaces the recorded GPS altitude with SRTM terrain data
+from the [Open-Elevation API](https://open-elevation.com) and recomputes gain and
+loss. The elevation chart and summary stats both update. Best for GPS-only devices
+(no barometric sensor) where the recorded altitude is noisy.
+
+> **Note:** Both corrections require a GPS track (activities marked *No GPS* cannot be
+> corrected). The DEM option uses ~30 m resolution terrain data; very short or indoor
+> activities see little improvement from DEM correction.
 
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