design: Themes/Tokens.xaml — one place a raw value may live, guarded by DESIGN.md

Tokens.xaml carries every colour, type size, spacing step, radius and motion duration from
DESIGN.md; App.xaml merges it BEFORE Controls.xaml so styles can resolve tokens.

Guard (DesignTokensTests, 4 facts): the test PARSES DESIGN.md — the colour tables under
"## Colour" are the contract — and fails if a documented token has no brush, if the hex
disagrees, if the meter bands drift, if Tokens ever merges after Controls, or if the retired
brand red #e94560 comes back as a token. Adding a colour to the standard now requires
implementing it, and changing the theme without the document is caught.

Inter/JetBrains Mono are declared as token families with system fallbacks; the bundled font
assets are the next unit.

No surface consumes tokens yet — this unit is the foundation only.

App.xaml

@@ -1,13 +1,16 @@
-<Application x:Class="ytLive.App"
-             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
-             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-             xmlns:local="clr-namespace:ytLive"
-             StartupUri="MainWindow.xaml">
-    <Application.Resources>
-        <ResourceDictionary>
-            <ResourceDictionary.MergedDictionaries>
-                <ResourceDictionary Source="Themes/Controls.xaml"/>
-            </ResourceDictionary.MergedDictionaries>
-        </ResourceDictionary>
-    </Application.Resources>
-</Application>
+<Application x:Class="ytLive.App"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:local="clr-namespace:ytLive"
+             StartupUri="MainWindow.xaml">
+    <Application.Resources>
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <!-- Design tokens first: every style below resolves its colours, sizes and
+                     radii from them (DESIGN.md is the source of truth). -->
+                <ResourceDictionary Source="Themes/Tokens.xaml"/>
+                <ResourceDictionary Source="Themes/Controls.xaml"/>
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
+    </Application.Resources>
+</Application>