feat(scoreboard): unit 10 — the polling policy (quiet by default)

ScoreboardCadence.Due(phase, lastAnalytics, lastSubscribers, now) is the creator's rule as a pure
function: watch hours ONLY during a live stream, every 30 minutes (Analytics answers in days and
test/record sessions have no public audience, so querying then is pure waste); subscribers whenever
the app is working, every 60 s; and nothing at all when idle.

Deliberately returns 'no poll' rather than 'analytics only' — a poll kind the poller cannot perform
is a bug waiting for a caller.

Remaining for the scoreboard: the HTTP providers behind this policy (watch hours via Analytics
reports.query, subscribers via channels.list) and the timer that calls Due(). Recorded in HANDOFF.

ytLive.Tests/ScoreboardCadenceTests.cs

@@ -0,0 +1,39 @@
+using System;
+using Xunit;
+using ytLive.Models;
+
+namespace ytLive.Tests;
+
+/// <summary>The quiet-by-default rule: watch hours only during a live stream (Analytics is a daily
+/// figure and test/record sessions have no public audience), subscribers whenever the app is
+/// working, and silence when idle.</summary>
+public class ScoreboardCadenceTests
+{
+    [Fact]
+    public void Analytics_polls_only_when_live_and_subscribers_poll_during_any_session()
+    {
+        var now = new DateTime(2026, 10, 30, 12, 0, 0, DateTimeKind.Utc);
+        var old = now.AddHours(-2);
+
+        // Idle: silent, even if everything is stale.
+        Assert.Equal(ScoreboardPoll.None, ScoreboardCadence.Due(SessionPhase.Idle, old, old, now));
+
+        // Live with stale figures: both.
+        Assert.Equal(ScoreboardPoll.Both, ScoreboardCadence.Due(SessionPhase.Live, old, old, now));
+
+        // Test / recording: subscribers only — no public audience, so no analytics.
+        Assert.Equal(ScoreboardPoll.SubscribersOnly, ScoreboardCadence.Due(SessionPhase.Test, old, old, now));
+        Assert.Equal(ScoreboardPoll.SubscribersOnly, ScoreboardCadence.Due(SessionPhase.Recording, old, old, now));
+
+        // Fresh figures: nothing is due.
+        Assert.Equal(ScoreboardPoll.None, ScoreboardCadence.Due(SessionPhase.Live, now, now, now));
+
+        // Analytics alone never polls (it is not a kind the poller knows).
+        Assert.Equal(ScoreboardPoll.None,
+            ScoreboardCadence.Due(SessionPhase.Live, old, now.AddSeconds(-10), now));
+
+        // Subscribers alone mid-window.
+        Assert.Equal(ScoreboardPoll.SubscribersOnly,
+            ScoreboardCadence.Due(SessionPhase.Live, now, old, now));
+    }
+}