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.

beefy_player/palettes.py

@@ -75,6 +75,32 @@ PALETTES: dict[str, tuple[str, ...]] = {
     "mono": ("bright_black", "white", "bright_white"),
 }
 
+#: The stock names from dark to bright.  These are xterm's default values in
+#: Rec. 601 luma order — the closest thing to a stock answer.  A theme may
+#: disagree about the exact numbers, but the ordering is what the frame fades
+#: need: a ladder is a *hue* walk, so where it ends says nothing about which
+#: end is brighter.
+BRIGHTNESS: tuple[str, ...] = (
+    "black",
+    "blue",
+    "red",
+    "bright_red",
+    "magenta",
+    "bright_magenta",
+    "bright_blue",
+    "green",
+    "bright_black",
+    "cyan",
+    "bright_green",
+    "bright_cyan",
+    "yellow",
+    "white",
+    "bright_yellow",
+    "bright_white",
+)
+
+_RANK = {name: rank for rank, name in enumerate(BRIGHTNESS)}
+
 #: Cycle order for the palette key.
 PALETTE_ORDER: tuple[str, ...] = tuple(PALETTES)
 
@@ -86,3 +112,8 @@ def palette_stops(name: str | None) -> tuple[str, ...]:
     """The ladder for ``name``, falling back to the default palette."""
     key = (name or "").strip().lower()
     return PALETTES.get(key, PALETTES[DEFAULT_PALETTE])
+
+
+def brightness(colour: str) -> int:
+    """How bright a stock name is: 0 (black) to 15 (bright white)."""
+    return _RANK.get(colour.strip().lower(), 0)