Merge the Trace rain tier into Very Light (#212)

The rain-intensity scale had a Trace tier (below the 1st percentile of a place's
rain days) sitting under Very Light — a sliver category that mostly showed 1% and
crowded the distribution strip. Fold it into Very Light, which now bottoms out the
scale at 0, so the lightest measurable rain reads as Very Light.

- grading.py: RAIN_BANDS drops the Trace band; Very Light's floor goes 1 -> 0. The
  detail-view ladder derives from the table, so it follows automatically.
- shared.js: SCALE_RAIN drops the Trace row; Very Light's range becomes "<10".

Eight rain tiers now instead of nine; the strip shows one fewer column. The wet-1
colour token is kept (unreferenced by new gradings) so any day still cached with
the old class renders until the derived store recomputes.

Claude-Session: https://claude.ai/code/session_013dRZmX9D3JEntfMKWMTWZ8

grading.py

@@ -47,7 +47,7 @@ TEMP_BANDS = [
 
 # Precipitation is graded ONLY among days that actually rained (>= RAIN_THRESHOLD)
 # in the seasonal window — a "rain percentile". Rain is one-directional (heavier =
-# more extreme), so these 9 tiers are sequential light->heavy, using the SAME cut
+# more extreme), so these 8 tiers are sequential light->heavy, using the SAME cut
 # points as temperature. Dry days are handled separately (the "dry" class, colored
 # by dry streak in the UI). Same [lower, upper) convention as TEMP_BANDS.
 RAIN_BANDS = [
@@ -58,8 +58,8 @@ RAIN_BANDS = [
     (40, "Moderate", "wet-5"),      # 40-60
     (25, "Light–Mod", "wet-4"),     # 25-40
     (10, "Light", "wet-3"),         # 10-25
-    (1, "Very Light", "wet-2"),     # 1-10
+    (0, "Very Light", "wet-2"),     # <10   the lightest measurable rain (was two
-    (0, "Trace", "wet-1"),          # <1    lightest measurable rain
+                                    #       tiers, Very Light + Trace, now merged)
 ]
 
 

tests/test_grading.py

@@ -81,6 +81,18 @@ def test_rain_with_no_historical_rain_days_is_extreme():
     assert g["percentile"] == 100.0 and g["class"] == "wet-9"
 
 
+def test_lightest_rain_day_grades_very_light_not_trace():
+    # The former "Trace" tier (<1st percentile of rain days) was merged into
+    # "Very Light", which now bottoms out the scale at 0.
+    assert "Trace" not in [b[1] for b in grading.RAIN_BANDS]
+    assert grading.RAIN_BANDS[-1] == (0, "Very Light", "wet-2")
+    rain = np.arange(1, 201, dtype=float) / 100.0     # 200 rain days: 0.01 .. 2.00
+    samples = np.concatenate([np.zeros(20), rain])
+    g = grading._grade_precip(samples, 0.01)           # the very lightest -> <1 pct
+    assert g["percentile"] < 1
+    assert g["grade"] == "Very Light" and g["class"] == "wet-2"
+
+
 # ---- dry streaks ---------------------------------------------------------------
 
 def test_dry_streaks_walk():