feat(panel): derived_bg en PanelFrameResult

Añade derived_bg al resultado de drawPanelFrame para que los paneles
puedan usar el color exacto del fondo para las tablas (mimetismo visual).

Las tablas deben usar frame_result.derived_bg como row_normal para
integrarse visualmente con el panel.

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

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

src/core/context.zig

@@ -728,6 +728,9 @@ pub const Context = struct {
         animating: bool,
         /// False if panel was suppressed - caller should return immediately
         should_draw: bool,
+        /// The actual background color being drawn (for table mimetism)
+        /// Tables should use this as row_normal to blend seamlessly with panel
+        derived_bg: Style.Color,
     };
 
     /// Draw a complete panel frame with focus transition and 3D effects.
@@ -832,7 +835,12 @@ pub const Context = struct {
         }
 
         // should_draw = false indica que el panel debe saltar operaciones costosas (BD, widgets)
-        return .{ .animating = animating, .should_draw = !burst_suppressed };
+        // derived_bg = color actual del fondo (para mimetismo de tablas)
+        return .{
+            .animating = animating,
+            .should_draw = !burst_suppressed,
+            .derived_bg = transition.current,
+        };
     }
 
     /// Resize the context