Elevation: skip near-zero dropout values mid-recording

Devices (Apple Watch, some GPS units) record 0.0 when they lose barometric/GPS
lock mid-activity. The old accumulation committed these as real sea-level points,
inflating both gain and loss by the current elevation (e.g. 792m dropout on the
Cosmo Walk added ~1584m of phantom gain+loss).

Fix: skip any elevation value < 1.0m when the current committed elevation is
significantly above zero (> threshold). Gradual legitimate descents to sea level
are unaffected because intermediate values are committed along the way.

Add --recompute-elevation flag to bincio render to backfill existing activities.

bincio/extract/metrics.py

@@ -422,6 +422,11 @@ def _elevation(
     gain = loss = 0.0
     committed = elevations[start]
     for e in elevations[start + 1:]:
+        # Skip near-zero values that appear mid-recording while we are at a
+        # significant elevation — these are sensor dropouts (device lost GPS/
+        # barometric lock), not genuine sea-level crossings.
+        if abs(e) < 1.0 and abs(committed) > threshold:
+            continue
         diff = e - committed
         if abs(diff) >= threshold:
             if diff > 0: