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/models.py

@@ -57,3 +57,7 @@ class ParsedActivity:
     strava_id: Optional[str] = None
     privacy: Optional[str] = None   # "public", "private", or None (caller decides)
     laps: list[LapData] = field(default_factory=list)
+    # "barometric" = device has a barometric altimeter (FIT enhanced_altitude present)
+    # "gps"        = altitude derived from GPS triangulation (GPX, TCX, FIT altitude-only)
+    # "unknown"    = source not detected (treated as gps for threshold purposes)
+    altitude_source: str = "unknown"