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.

ViewModels/MainViewModel.Streaming.cs

@@ -488,7 +488,11 @@ public partial class MainViewModel : ViewModelBase
             }
 
             if (SetProperty(ref _selectedQuality, value))
+            {
                 ApplyStreamQuality(value);
+                OnPropertyChanged(nameof(RatedMbps));
+                OnPropertyChanged(nameof(RateHint));
+            }
         }
     }
 
@@ -505,7 +509,7 @@ public partial class MainViewModel : ViewModelBase
     private void ApplyStreamQuality(QualityOption quality)
     {
         var health = CurrentHealth;
-        health.CurrentBitrate = quality.Bitrate;
+        health.CurrentBitrate = EffectiveQuality(quality).Bitrate;
         health.FPS = quality.Fps;
         OnPropertyChanged(nameof(CurrentHealth));