feat(themes): arm crossfade state in ThemeRegistry.Switch

Three new private fields plus TryGetActiveCrossfade entry-point, plus
SwitchSilent variant for the plugin-load init path. ArmCrossfade
captures a value-copy of the active AbgrCache and stamps TickCount64;
mid-crossfade Switch composes the current lerped state as the next
fade origin so back-to-back theme switches stay smooth.

Same-slug Switch is a no-op (no identity-crossfade).

HellionChat/Themes/ThemeRegistry.cs

@@ -32,6 +32,16 @@ public sealed class ThemeRegistry
     private long _lastActiveStampCheckMs = -ActiveStampPollIntervalMs;
     private DateTime _lastActiveStamp = DateTime.MinValue;
 
+    // PM-1 crossfade state. Switch() captures the previous AbgrCache as a
+    // VALUE-COPY (not a Theme reference) -- the built-in singletons share
+    // their RecomputeAbgrCache identity, so a reference would mutate
+    // alongside the new active. _crossfadeStartTickMs == long.MinValue
+    // means "no crossfade armed yet"; the field stays MinValue after
+    // SwitchSilent so the plugin-load init-path does not trigger a fade.
+    private ThemeAbgrCache? _previousAbgrSnapshot;
+    private long _crossfadeStartTickMs = long.MinValue;
+    private const int CrossfadeDurationMs = 300;
+
     public ThemeRegistry(string? customThemesDir = null, ILogger<ThemeRegistry>? logger = null)
     {
         _logger = logger;
@@ -87,6 +97,13 @@ public sealed class ThemeRegistry
     // a state where _active and Get(_active.Slug) disagree.
     public void Switch(string slug)
     {
+        // Same-slug switch is a no-op -- avoids a 300ms identity-crossfade
+        // when the user re-selects the active theme in the picker.
+        if (string.Equals(_active.Slug, slug, StringComparison.OrdinalIgnoreCase))
+            return;
+
+        ArmCrossfade();
+
         if (_builtIns.TryGetValue(slug, out var builtin))
         {
             _active = builtin;
@@ -115,6 +132,84 @@ public sealed class ThemeRegistry
         _activeCustomPath = null;
     }
 
+    // SwitchSilent is the plugin-load init path -- identical to Switch
+    // but does NOT arm the crossfade state. Called from
+    // ThemeRegistryInitHostedService.StartAsync so opening the plugin
+    // does not produce a 300ms fade from the default theme to the user's
+    // saved theme.
+    public void SwitchSilent(string slug)
+    {
+        if (string.Equals(_active.Slug, slug, StringComparison.OrdinalIgnoreCase))
+            return;
+
+        if (_builtIns.TryGetValue(slug, out var builtin))
+        {
+            _active = builtin;
+            _active.RecomputeAbgrCache();
+            _activeCustomPath = null;
+            return;
+        }
+
+        var customTheme = LoadCustomBySlug(slug, out var customPath);
+        if (customTheme is not null)
+        {
+            _active = customTheme;
+            _active.RecomputeAbgrCache();
+            _activeCustomPath = customPath;
+            _lastActiveStamp = DateTime.MinValue;
+            return;
+        }
+
+        _active = _builtIns[DefaultSlug];
+        _active.RecomputeAbgrCache();
+        _activeCustomPath = null;
+    }
+
+    // Captures the AbgrCache snapshot that PushGlobal should fade FROM.
+    // If a crossfade is already mid-flight (second Switch within 300ms),
+    // the current lerped state replaces the snapshot -- the next fade
+    // starts from where we currently are, not from the original "from".
+    private void ArmCrossfade()
+    {
+        var now = Environment.TickCount64;
+        ThemeAbgrCache snapshot;
+        if (
+            _previousAbgrSnapshot.HasValue
+            && _crossfadeStartTickMs != long.MinValue
+            && now - _crossfadeStartTickMs < CrossfadeDurationMs
+        )
+        {
+            var t = (float)(now - _crossfadeStartTickMs) / CrossfadeDurationMs;
+            snapshot = ThemeAbgrCacheLerp.Lerp(_previousAbgrSnapshot.Value, _active.AbgrCache, t);
+        }
+        else
+        {
+            snapshot = _active.AbgrCache;
+        }
+
+        _previousAbgrSnapshot = snapshot;
+        _crossfadeStartTickMs = now;
+    }
+
+    // Returns the lerped AbgrCache while the crossfade is active.
+    // PushGlobal reads this once per frame; outside the 300ms window
+    // it short-circuits via the TickCount64 delta so the per-frame
+    // overhead is a couple of integer comparisons.
+    public bool TryGetActiveCrossfade(out ThemeAbgrCache lerped)
+    {
+        lerped = default;
+        if (_crossfadeStartTickMs == long.MinValue || !_previousAbgrSnapshot.HasValue)
+            return false;
+
+        var elapsed = Environment.TickCount64 - _crossfadeStartTickMs;
+        if (elapsed >= CrossfadeDurationMs)
+            return false;
+
+        var t = (float)elapsed / CrossfadeDurationMs;
+        lerped = ThemeAbgrCacheLerp.Lerp(_previousAbgrSnapshot.Value, _active.AbgrCache, t);
+        return true;
+    }
+
     // 1Hz-throttled disk-stat on the currently active custom theme file.
     // When the file's LastWriteTime moves forward (editor save), reload the
     // theme via Get() so the user sees the edit immediately without