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/parsers/fit.py

@@ -20,6 +20,8 @@ class FitParser:
         sub_sport: str | None = None
         device: str | None = None
 
+        has_baro_alt = False  # True if any record used enhanced_altitude
+
         with fitdecode.FitReader(io.BytesIO(raw_bytes)) as fit:
             for frame in fit:
                 if not isinstance(frame, fitdecode.FitDataMessage):
@@ -60,7 +62,9 @@ class FitParser:
                     # enhanced_altitude is written by barometric altimeters (most
                     # modern Garmins). Fall back to GPS-derived altitude if absent.
                     _alt = _get(frame, "enhanced_altitude")
-                    if _alt is None:
+                    if _alt is not None:
+                        has_baro_alt = True
+                    else:
                         _alt = _get(frame, "altitude")
 
                     dp = DataPoint(
@@ -100,6 +104,8 @@ class FitParser:
         if not points:
             raise ValueError(f"No record messages found in {path.name}")
 
+        altitude_source = "barometric" if has_baro_alt else "gps"
+
         return ParsedActivity(
             points=points,
             sport=sport,
@@ -109,6 +115,7 @@ class FitParser:
             laps=laps,
             source_file=path.name,
             source_hash="",
+            altitude_source=altitude_source,
         )
 
 

bincio/extract/parsers/gpx.py

@@ -53,6 +53,7 @@ class GpxParser(BaseParser):
             started_at=started_at,
             source_file=path.name,
             source_hash="",  # set by factory
+            altitude_source="gps",
         )
 
 

bincio/extract/parsers/tcx.py

@@ -83,6 +83,7 @@ class TcxParser:
             started_at=points[0].timestamp,
             source_file=path.name,
             source_hash="",
+            altitude_source="gps",
         )