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.

MainWindow.xaml

@@ -145,6 +145,20 @@
                                   ItemContainerStyle="{StaticResource YtQualityComboBoxItem}"
                                   ToolTip="{Binding ResolutionHelp}"
                                   IsEnabled="{Binding CanChangeQuality}"/>
+                        <!-- The creator's own rate: the tier sets the geometry, this sets the bits.
+                             Guidance lives in the tooltip (YouTube's numbers, from RateLadder). -->
+                        <Grid Margin="0,6,0,0">
+                            <Grid.ColumnDefinitions>
+                                <ColumnDefinition Width="*"/>
+                                <ColumnDefinition Width="Auto"/>
+                            </Grid.ColumnDefinitions>
+                            <TextBox x:Name="RatedMbpsBox" Grid.Column="0" Style="{StaticResource YtTextBoxCompact}"
+                                     Text="{Binding RatedMbps, UpdateSourceTrigger=LostFocus, StringFormat={}{0:0.#}}"
+                                     ToolTip="{Binding RateHint}"
+                                     IsEnabled="{Binding CanChangeQuality}"/>
+                            <TextBlock Grid.Column="1" Text="Mbps" Margin="6,0,0,0" VerticalAlignment="Center"
+                                       Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="12"/>
+                        </Grid>
                     </StackPanel>
                 </Grid>
             </Border>