design: convert Themes/Controls.xaml to tokens (surface 1 of the sweep)

The theme file that every control inherits is now token-driven: 73 hardcoded colours became
Brush.* references, and the two role changes the standard requires landed here first.

- Red leaves the chrome. The seven #e94560 sites (base button, focus borders, checked box and
  segment backgrounds, the selected-layer accent strip) were all interactive/selection roles,
  so they are blue now. Red stays for live/record only.
- The slider was green: its fill and gloss knob are blue now, with the track on
  SurfaceContainerLowest. The old green fill predated the standard.
- The pill toggle was ON = green, OFF = red — a settings control using the live colour. It is
  now OFF = neutral surface, ON = blue.
- Added scrim-soft/scrim-strong tokens (#22FFFFFF/#26FFFFFF) for the existing white washes, and
  Color.* resources for the two gradients (a GradientStop needs a Color, not a Brush).

Guard: DesignTokensTests gains Converted_surfaces_carry_no_raw_colour — a declared list of
converted files, so each sweep commit is guarded as it lands instead of waiting for one global
grep at the end. Pure white/black highlights are the only allowed literals.

No layout, spacing or behaviour change: this unit is colour only.

ytLive.Tests/DesignTokensTests.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Text.RegularExpressions;
 using System.Xml.Linq;
 using Xunit;
 
@@ -67,6 +68,28 @@ public class DesignTokensTests
             "Themes/Tokens.xaml must merge BEFORE Themes/Controls.xaml, or styles cannot resolve a token");
     }
 
+    [Fact]
+    public void Converted_surfaces_carry_no_raw_colour()
+    {
+        // The sweep converts one surface per commit; each adds its file here, so every step is
+        // guarded on its own instead of waiting for a global grep at the end. Pure white and black
+        // are allowed (gloss highlights, knockout masks) — everything else must be a token.
+        string[] converted = { "Themes/Controls.xaml" };
+
+        foreach (var file in converted)
+        {
+            var text = SourceAssert.Text(file.Split('/'));
+            var raw = Regex.Matches(text, @"#[0-9a-fA-F]{6,8}")
+                .Select(m => m.Value)
+                .Where(v => v is not "#ffffff" and not "#000000" and not "#FFFFFF" and not "#000000")
+                .Distinct(StringComparer.OrdinalIgnoreCase)
+                .ToList();
+
+            Assert.True(raw.Count == 0,
+                $"{file} still hardcodes {string.Join(", ", raw)} — resolve a Brush.* token instead");
+        }
+    }
+
     [Fact]
     public void The_retired_brand_red_is_not_a_token()
     {
@@ -100,7 +123,8 @@ public class DesignTokensTests
             // The token cell may carry a note after the backticked name("`tertiary` (electric blue)").
             var token = Backticked(cells[0]);
             var hex = Backticked(cells[1]);
-            if (!hex.StartsWith("#", StringComparison.Ordinal) || hex.Length != 7) continue;
+            // 6-digit RGB or 8-digit ARGB (the scrim washes carry alpha).
+            if (!hex.StartsWith("#", StringComparison.Ordinal) || hex.Length is not (7 or 9)) continue;
             result[token] = hex;
         }