Fix pre-existing test failures in test_writer and test_metrics

test_writer: _dummy_metrics() and test_build_summary_required_fields were
missing np_power_w=None after the field was added to ComputedMetrics.

test_metrics: the leading-zero elevation heuristic fired on a single 0.0
start value, incorrectly skipping the first legitimate elevation step.
Guard now requires at least 2 consecutive near-zero leading values before
activating the Apple Watch lock-acquisition workaround.

tests/test_writer.py

@@ -67,7 +67,7 @@ def _dummy_metrics(**overrides):
         elevation_gain_m=100.0, elevation_loss_m=95.0,
         avg_speed_kmh=10.0, max_speed_kmh=20.0,
         avg_hr_bpm=None, max_hr_bpm=None,
-        avg_cadence_rpm=None, avg_power_w=None, max_power_w=None,
+        avg_cadence_rpm=None, avg_power_w=None, np_power_w=None, max_power_w=None,
         bbox=None, start_latlng=None, end_latlng=None,
         mmp=None, best_efforts=None, best_climb_m=None,
     )
@@ -208,6 +208,7 @@ def test_build_summary_required_fields():
         max_hr_bpm=None,
         avg_cadence_rpm=None,
         avg_power_w=None,
+        np_power_w=None,
         max_power_w=None,
         bbox=None,
         start_latlng=None,