metrics: replace naive elevation accumulation with hysteresis dead-band

GPS jitter and barometric quantization noise caused systematic overestimation
of elevation gain — in extreme cases 100% of reported gain was sub-1m noise.

Implements source-aware hysteresis: elevation is only committed when it
deviates from the last committed value by ≥5m (barometric) or ≥10m (GPS/GPX/TCX).

- ParsedActivity gains `altitude_source` field ("barometric"/"gps"/"unknown")
- FIT parser sets "barometric" when enhanced_altitude is present, else "gps"
- GPX and TCX parsers always set "gps"
- metrics._elevation() uses the threshold matching the source
- 5 new parametric tests covering flat GPS noise, threshold differences, and real climbs

bincio/extract/metrics.py

@@ -70,7 +70,7 @@ def compute(activity: ParsedActivity) -> ComputedMetrics:
 
     duration_s = _duration(pts)
     distance_m, moving_time_s, avg_speed_kmh, max_speed_kmh = _gps_stats(pts)
-    gain, loss = _elevation(pts)
+    gain, loss = _elevation(pts, activity.altitude_source)
     avg_hr, max_hr = _hr_stats(pts)
     avg_cad = _avg_nonnull([p.cadence_rpm for p in pts])
     avg_pow = _avg_nonnull([p.power_w for p in pts])
@@ -347,17 +347,40 @@ def _duration(pts: list[DataPoint]) -> Optional[int]:
     return int((pts[-1].timestamp - pts[0].timestamp).total_seconds())
 
 
-def _elevation(pts: list[DataPoint]) -> tuple[Optional[float], Optional[float]]:
+# Hysteresis thresholds per altitude source.
+# Only commit a new elevation when it differs from the last committed value by
+# at least this amount, filtering out GPS noise and barometric quantization steps.
+_ELEVATION_THRESHOLD: dict[str, float] = {
+    "barometric": 5.0,   # barometric altimeter: smaller steps are real
+    "gps":        10.0,  # GPS altitude: noisier, needs wider dead-band
+    "unknown":    10.0,  # treat unknown as GPS to be conservative
+}
+
+
+def _elevation(
+    pts: list[DataPoint],
+    altitude_source: str = "unknown",
+) -> tuple[Optional[float], Optional[float]]:
+    """Hysteresis-based elevation accumulation.
+
+    Only commits a new elevation when it differs from the last committed value
+    by at least the source-specific threshold, filtering GPS jitter and
+    barometric quantization noise that would otherwise inflate the gain figure.
+    """
     elevations = [p.elevation_m for p in pts if p.elevation_m is not None]
     if len(elevations) < 2:
         return None, None
+    threshold = _ELEVATION_THRESHOLD.get(altitude_source, 10.0)
     gain = loss = 0.0
-    for a, b in zip(elevations, elevations[1:]):
-        diff = b - a
-        if diff > 0:
-            gain += diff
-        else:
-            loss += diff
+    committed = elevations[0]
+    for e in elevations[1:]:
+        diff = e - committed
+        if abs(diff) >= threshold:
+            if diff > 0:
+                gain += diff
+            else:
+                loss += diff
+            committed = e
     return gain, loss