feat(quality): the creator sets the rate — per tier, persisted, clamped only at YouTube's floor

The tier table cannot know a creator's connection, so it should not pretend to. The
Mbps box under the picker is theirs: the tier chooses geometry and frame rate, the
box chooses bits. Stored per tier (`Video.Rate.<tier>`), so tuning 1080p60 does not
touch 720p30, and none means "the shipped default" rather than zero.

Clamped at the tier's YouTube minimum (`RateLadder`), because a target under it
cannot hold the resolution; no ceiling otherwise, because a wrong guess is exactly
what live step-down exists to catch, and a creator who types 25 on a fibre line
should be allowed to find out. The tooltip carries YouTube's number for the tier.

`MainViewModel.EffectiveQuality` is the single substitution point, and the encoder
options, the step-down policy and the clip buffer all read it — so a rated tier can
never stream at the shipped default because one call site was missed.

One ordering trap found by the test: the constructor resolves the default tier
before the layout store is open, so a read at that point must not cache, or the
shipped default shadows the creator's stored rate for the whole run.

670 pass, 0 warnings.

Services/LayoutStore.Settings.cs

@@ -27,6 +27,26 @@ public partial class LayoutStore : IDisposable
     public void SaveStickyAlertsUntilDismissed(bool enabled)
         => UpsertSetting("Notifications.StickyUntilDismissed", enabled ? "1" : "0");
 
+    /// <summary>The creator's own video rate for one tier, in Mbps, or null to use the shipped default.
+    /// Per tier because each has its own ceiling; the rate is a target the encoder is asked to hold,
+    /// so a stored number means the same thing on every run (CBR).</summary>
+    public double? LoadRatedMbps(string tierDisplay)
+        => double.TryParse(GetSetting($"Video.Rate.{tierDisplay}"), out var mbps) && mbps > 0 ? mbps : null;
+
+    public void SaveRatedMbps(string tierDisplay, double? mbps)
+    {
+        if (mbps is not > 0)
+        {
+            using var delete = _connection.CreateCommand();
+            delete.CommandText = "DELETE FROM Settings WHERE Key = $key;";
+            delete.Parameters.AddWithValue("$key", $"Video.Rate.{tierDisplay}");
+            delete.ExecuteNonQuery();
+            return;
+        }
+
+        UpsertSetting($"Video.Rate.{tierDisplay}", mbps.Value.ToString("0.#"));
+    }
+
     private static bool IsSupportedNotificationPosition(NotificationPosition position)
         => position is NotificationPosition.TopLeft or NotificationPosition.TopRight
             or NotificationPosition.CenterLeft or NotificationPosition.CenterRight