feat(step-down): the bitrate floor is the tier's YouTube minimum

Step-down used a flat 3 Mbps (4 for vertical) as its floor — a number that had
nothing to do with the tier being streamed. With CBR holding the target, that
floor is a real picture-quality floor, so it has to follow YouTube's published
H.264 table (support.google.com/youtube/answer/2853702): 6 at 1080p60, 5 at
1080p30, 3 at 720p.

Models/RateLadder is that table as pure policy. Vertical is not a separate row:
1080x1920 and 1920x1080 are the same pixel count, so the frame rate alone picks
the band. StepDownPolicy takes the floor as an optional constructor argument and
refuses one above the starting rate, so an unusual start cannot begin under its
own floor. Omitted, the historical 3/4 floor still applies, which keeps the
existing policy tests meaningful and lets a caller opt in.

One consequence worth knowing: the three-step budget can stop above the floor
(12 → 9.6 → 7.7 → 6.2 from a 20% loss), so the floor is a limit, not a target.

669 pass, 0 warnings.

ViewModels/MainViewModel.StepDown.cs

@@ -26,7 +26,8 @@ public partial class MainViewModel
         var quality = OutputProfile.Resolve(SelectedQuality, QualityOptions, HasProOutput);
         _encoderTierOverride = null;
         _stepDownPlanner = new StepDownPlanner(
-            new StepDownPolicy(quality.Width, quality.Height, quality.Fps, quality.Bitrate),
+            new StepDownPolicy(quality.Width, quality.Height, quality.Fps, quality.Bitrate,
+                floorBitrateMbps: RateLadder.For(quality).Minimum),
             _sessionStartedAtUtc ?? DateTime.UtcNow);
         _framePump.StatsReported += OnPumpStatsReported;
     }