terminal: default render state foreground to white

Fixes #11704

The RenderState empty initializer set both background and foreground
to the default RGB value of black (0, 0, 0), making text unreadable
when a caller has not explicitly configured terminal colors via
DynamicRGB. This is the common case for libghostty consumers.

Default the foreground to white so that the initial render state
provides readable white-on-black text out of the box.

Long term we also need to expose setting the default colors for a
Terminal instance but this is a workable fix in the mean time.

src/terminal/render.zig

@@ -98,8 +98,8 @@ pub const RenderState = struct {
         .rows = 0,
         .cols = 0,
         .colors = .{
-            .background = .{},
-            .foreground = .{},
+            .background = .{ .r = 0, .g = 0, .b = 0 },
+            .foreground = .{ .r = 0xff, .g = 0xff, .b = 0xff },
             .cursor = null,
             .palette = color.default,
         },