fix(growth): rank the rate board by a smoothed score

Show the real percentage on the card, but sort with delta / (baseline + prior) so a small baseline cannot dominate. Same-day publishes stay off the rate board.

tests/test_analytics.py

@@ -12,6 +12,7 @@ from reaction_board.analytics import (  # noqa: E402
     METRICS,
     age_days,
     compute_growth,
+    smoothed_growth_rate,
     content_hours,
     eb_rate,
     effective_shrink_minutes,
@@ -161,6 +162,19 @@ def test_compute_growth():
     assert negative.per_day == -5.0
 
 
+def test_smoothed_growth_rate():
+    # 80→512：真实增速 5.4；prior=500 时排序分 432/580
+    assert abs(smoothed_growth_rate(432, 80, 500) - 432 / 580) < 1e-12
+    assert abs(smoothed_growth_rate(432, 5000, 500) - 432 / 5500) < 1e-12
+    assert abs(smoothed_growth_rate(432, 80, 1000) - 432 / 1080) < 1e-12
+    # prior=0 时排序分退回真实相对增速（基线 ≥ 1）
+    assert smoothed_growth_rate(432, 80, 0) == 432 / 80
+    assert smoothed_growth_rate(None, 80, 500) is None
+    assert smoothed_growth_rate(432, None, 500) is None
+    assert smoothed_growth_rate(10, 0, 0) is None
+    assert smoothed_growth_rate(10, 80, -1) is None
+
+
 def test_normalize_cover_and_hosts():
     assert normalize_cover(None) is None
     assert normalize_cover("") is None
@@ -251,6 +265,7 @@ def main_test():
     test_score_percentiles()
     test_interaction_rate_thresholds()
     test_compute_growth()
+    test_smoothed_growth_rate()
     test_normalize_cover_and_hosts()
     test_content_efficiency_helpers()
     test_age_and_maturity()