feat(animation): ColorTransition para transiciones suaves de color

Transiciones Suaves (Acabado Espectacular mejora #4):
- Nuevo struct ColorTransition en animation.zig
- Interpola colores en ~200ms usando lerp
- Se inicializa automáticamente en primer uso
- Exportado en zcatgui.zig junto con HoverTransition

Uso: state.bg_transition.update(target_color, delta_ms)
     ctx.drawRect(..., state.bg_transition.current)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/render/animation.zig

@@ -678,6 +678,103 @@ pub const HoverTransition = struct {
     }
 };
 
+// =============================================================================
+// Color Transition (for panel backgrounds)
+// =============================================================================
+
+const Style = @import("../core/style.zig");
+
+/// Smooth color transition for panel backgrounds.
+/// Interpolates from current to target color over ~200ms.
+///
+/// Usage:
+/// ```zig
+/// pub const PanelState = struct {
+///     bg_transition: ColorTransition = .{},
+/// };
+///
+/// // In panel draw:
+/// const target_color = if (has_focus) focus_color else normal_color;
+/// state.bg_transition.update(target_color, ctx.delta_ms);
+/// const display_color = state.bg_transition.current;
+/// ctx.drawRect(..., display_color);
+/// ```
+pub const ColorTransition = struct {
+    /// Current displayed color
+    current: Style.Color = Style.Color.rgb(0, 0, 0),
+    /// Is initialized with a color?
+    initialized: bool = false,
+    /// Transition duration in milliseconds
+    duration_ms: f32 = 200.0,
+
+    const Self = @This();
+
+    /// Update transition towards target color
+    /// Returns true if color changed (for requesting redraw)
+    pub fn update(self: *Self, target: Style.Color, delta_ms: u64) bool {
+        // First call: snap to target immediately
+        if (!self.initialized) {
+            self.current = target;
+            self.initialized = true;
+            return true;
+        }
+
+        // Already at target?
+        if (self.current.r == target.r and
+            self.current.g == target.g and
+            self.current.b == target.b and
+            self.current.a == target.a)
+        {
+            return false;
+        }
+
+        // Calculate interpolation factor (chase towards target)
+        const t = @min(1.0, @as(f32, @floatFromInt(delta_ms)) / self.duration_ms);
+
+        // Interpolate each channel
+        self.current = Style.Color{
+            .r = lerpU8(self.current.r, target.r, t),
+            .g = lerpU8(self.current.g, target.g, t),
+            .b = lerpU8(self.current.b, target.b, t),
+            .a = lerpU8(self.current.a, target.a, t),
+        };
+
+        return true;
+    }
+
+    /// Reset to uninitialized state
+    pub fn reset(self: *Self) void {
+        self.initialized = false;
+    }
+};
+
+/// Interpolate between two u8 values
+fn lerpU8(a: u8, b: u8, t: f32) u8 {
+    const af: f32 = @floatFromInt(a);
+    const bf: f32 = @floatFromInt(b);
+    return @intFromFloat(af + (bf - af) * t);
+}
+
+test "ColorTransition basic" {
+    var trans = ColorTransition{};
+    const black = Style.Color.rgb(0, 0, 0);
+    const white = Style.Color.rgb(255, 255, 255);
+
+    // First update snaps to target
+    _ = trans.update(black, 16);
+    try std.testing.expect(trans.initialized);
+    try std.testing.expectEqual(@as(u8, 0), trans.current.r);
+
+    // Transition towards white over time
+    _ = trans.update(white, 100); // ~50% transition
+    try std.testing.expect(trans.current.r > 0);
+    try std.testing.expect(trans.current.r < 255);
+
+    // Full duration reaches target
+    _ = trans.update(white, 200);
+    try std.testing.expectEqual(@as(u8, 255), trans.current.r);
+}
+
 test "HoverTransition basic" {
     var hover = HoverTransition{};
 

src/zcatgui.zig

@@ -109,6 +109,8 @@ pub const lerp = render.animation.lerp;
 pub const lerpInt = render.animation.lerpInt;
 pub const Spring = render.animation.Spring;
 pub const SpringConfig = render.animation.SpringConfig;
+pub const HoverTransition = render.animation.HoverTransition;
+pub const ColorTransition = render.animation.ColorTransition;
 
 // Effects re-exports
 pub const Shadow = render.effects.Shadow;