feat: the flat looks of the newly relief-styled controls, and the alpha ColorSelector

cce-ui@969f873 gives ButtonStrip, FontSelector,
KeybindRecorder, ColorSelector, RangeSlider, ProgressBar, UsageBar and Trackpad the
relief idiom by default; the named slots now show that, and each gets a "(flat)"
variant exhibit for what `control_relief = 0` renders. ColorSelector also shows its
alpha swatch.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

 README.md   | 10 ++++++++--
 src/main.rs | 20 ++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 2869379..93f8959 100644
--- a/README.md
+++ b/README.md
@@ -29,8 +29,14 @@ Dropdown; `with_band`, `with_recessed(false)` and `with_readout` on Slider;
 TextBox; `with_vertical` on ButtonStrip, and the Paginator built on it), or a
 per-widget override of a config-wide style (`Toggle::with_slide`, the
 `style.control.toggle.style` look, next to `Slider::with_band` for
-`style.control.slider.style`). The four StatusDot statuses and the plain
-Label round it off. When a widget gains a style, it gains an exhibit here.
+`style.control.slider.style`). The controls whose default look is worked
+into the plate — ButtonStrip's well with its raised selected segment,
+FontSelector's flush trough, the wells of KeybindRecorder, ColorSelector's
+hex field, RangeSlider, ProgressBar, UsageBar and Trackpad — show their flat
+look too (`with_recessed(false)` / `with_raised(false)`, what
+`control_relief = 0` renders), and ColorSelector its alpha swatch. The four
+StatusDot statuses and the plain Label round it off. When a widget gains a
+style, it gains an exhibit here.
 
 Every exhibit is drawn at its toolkit default size — the control's configured
 `style.control.<name>.height`, or the intrinsic size the widget declares — so
diff --git a/src/main.rs b/src/main.rs
index 97eaa25..06472e5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -735,6 +735,10 @@ fn variant_exhibits() -> Vec<Exhibit> {
     let slh = cce_ui::layout::slider_height();
     let tbh = cce_ui::layout::textbox_height();
     let ddh = cce_ui::layout::dropdown_height();
+    let fsh = cce_ui::layout::font_selector_height();
+    let csh = cce_ui::layout::color_selector_height();
+    let rsh = cce_ui::layout::rangeslider_height();
+    let pbh = cce_ui::layout::progressbar_height();
     // A StatusDot is 12px square; its exhibit is as wide as its label.
     const DOT_W: f32 = 150.0;
     // A column of three rotated tabs, and the sidebar width a vertical strip is drawn for.
@@ -780,6 +784,22 @@ fn variant_exhibits() -> Vec<Exhibit> {
         )
         .with_content_width(TAB_COLUMN_W),
         Exhibit::sized(Paginator::new(three()).with_label("Paginator"), W, TABS_H),
+        // The flat (relief-off) looks of the controls whose default is a well or a
+        // trough carved into the plate: what `control_relief = 0` renders.
+        Exhibit::sized(
+            Adapted::new(ButtonStrip::new(0.0, 0.0, W, bh).with_buttons(three()).with_selected(Some(0)).with_recessed(false))
+                .with_label("ButtonStrip (flat)"),
+            W,
+            bh,
+        ),
+        Exhibit::new(FontSelector::new("Sans".to_string()).with_raised(false).with_label("FontSelector (flat)"), W, fsh),
+        Exhibit::new(KeybindRecorder::new("ctrl+1".to_string()).with_recessed(false).with_label("KeybindRecorder (flat)"), W, tbh),
+        Exhibit::new(ColorSelector::new([64, 128, 255]).with_recessed(false).with_label("ColorSelector (flat)"), W, csh),
+        Exhibit::new(ColorSelector::new_rgba([64, 128, 255, 128]).with_label("ColorSelector (alpha)"), W, csh),
+        Exhibit::new(RangeSlider::new().with_recessed(false).with_label("RangeSlider (flat)"), W, rsh),
+        Exhibit::new(ProgressBar::new(0.43).with_recessed(false).with_label("ProgressBar (flat)"), W, pbh),
+        Exhibit::new(UsageBar::new(0.62).with_recessed(false).with_label("UsageBar (flat)"), W, pbh),
+        Exhibit::sized(Trackpad::new().with_recessed(false).with_label("Trackpad (flat)"), W, TABS_H),
         // Label: the plain text widget.
         Exhibit::new(Label::new("Label"), W, ddh),
         // StatusDot: the other three statuses.