docs(tools): note that ansi_art heroes must opt out of the gradient

tools/ansi_art.py

@@ -7,11 +7,18 @@ half.  Colours are quantised to the stock 16-colour ANSI palette on purpose —
 the rest of the UI never emits RGB, and a hero built from palette names still
 reads as the station's art in any terminal theme.
 
+Shipped catalogues no longer use image art: each one renders a wordmark from
+beefy_player/providers/wordmark.py, which the header repaints with the active
+palette gradient (RadioProvider.hero_gradient, on by default).  Art from this
+tool carries colours of its own, so a provider that uses it must set
+hero_gradient = False — otherwise the header repaints the art with the gradient
+and the palette names below are wasted.
+
 Usage:
 
     python tools/ansi_art.py logo.png --cols 22 --rows 7
-    python tools/ansi_art.py --url https://tilderadio.org/logos/tilderadio.png \\
-        --cols 22 --rows 7 --module beefy_player/providers/tilderadio/hero.py
+    python tools/ansi_art.py --url https://example.org/logo.png \\
+        --cols 22 --rows 7 --module beefy_player/providers/<slug>/hero.py
 
 ``--preview out.png`` draws what the cells will look like, which is much easier
 to judge than a wall of ▀ and ▄.
@@ -209,6 +216,10 @@ Regenerate with::
 Each row is rich markup; ``▀`` is a half block whose foreground is the top
 pixel and whose background is the pixel below it.  Colours are stock ANSI
 palette names, so the art keeps the terminal's own palette.
+
+The header paints every hero with the active palette gradient unless the
+provider opts out, so the class using this art must set
+``hero_gradient = False``.
 """
 
 from __future__ import annotations