fix(frame): start the corner fade at the brightest palette stop

A ladder is a hue walk, not a brightness walk, so painting the hot end
backwards put the dim colour at the corner.  Sort the ramp by stock
brightness and follow the pane height for the bottom-left glow.

tests/test_palettes.py

@@ -13,6 +13,7 @@ from beefy_player.palettes import (
     DEFAULT_PALETTE,
     PALETTE_ORDER,
     PALETTES,
+    brightness,
     palette_stops,
 )
 from beefy_player.widgets import frame
@@ -133,13 +134,27 @@ async def test_the_spectrum_rows_use_the_active_palette() -> None:
         assert colours(spectrum._row_palette(12, 0, False)) <= set(PALETTES["neon"])
 
 
-def test_the_corner_fade_walks_back_down_the_ladder() -> None:
+def test_the_corner_fade_starts_at_the_brightest_stop() -> None:
     stops = PALETTES["spectrum"]
-    assert frame.corner_fade(0, stops) == stops[-1]
-    assert frame.corner_fade(1, stops) == stops[-2]
-    assert frame.corner_fade(3, stops) == stops[-4]
-    # the fade is short: past four cells it is just the border grey
-    assert frame.corner_fade(4, stops) == frame.EDGE
+    ramp = frame.fade_ramp(stops)
+    assert frame.corner_fade(0, stops) == ramp[0]
+    assert frame.corner_fade(1, stops) == ramp[1]
+    # the fade is short: past the ramp it is just the border grey
+    assert frame.corner_fade(len(ramp), stops) == frame.EDGE
+    # spectrum's hot end is red, which is dimmer than the yellow before it;
+    # the corner still gets the bright colour
+    assert ramp[0] != stops[-1]
+    assert ramp[-1] == stops[-1]
+
+
+def test_every_palette_fades_from_bright_to_dim() -> None:
+    """No palette may put a brighter cell further from the corner."""
+    for name, stops in PALETTES.items():
+        ramp = frame.fade_ramp(stops)
+        assert ramp, name
+        ranks = [brightness(colour) for colour in ramp]
+        assert ranks == sorted(ranks, reverse=True), (name, ramp)
+        assert set(ramp) <= set(stops), name
 
 
 @pytest.mark.asyncio