docs: tick the drawn chevron

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/RUST.md

@@ -8019,12 +8019,15 @@ framework and by looking at the fixture running
   shaped-mask work (`.masked_by`, 38bf630). `raw_block` pans sideways
   again instead of clipping, so a long command is readable. Ticked there
   with the original symptom kept.
-- **Still open, and the one certain thing in her screenshot**: the
-  chevron. The mark is a font codepoint (U+25B8/25BE/25B4) and the
-  bundled fonts were removed on 2026-09-07, so the phone draws an empty
-  box and the desktop draws a dot. It needs a drawn mark -- a texture
-  generated at the size wanted, since iris has rects, text and textures
-  and no path primitive. Not done here.
+- **The one certain thing in her screenshot, now fixed**: the chevron.
+  The mark was a font codepoint (U+25B8/25BE/25B4) and the bundled fonts
+  were removed on 2026-09-07, so the phone drew an empty box and the
+  desktop a dot. `iris::widget::mark(dir, dp, colour)` draws it instead:
+  one oversampled, antialiased triangle rasterised into the ordinary
+  texture path and scaled into the box asked for, so iris needed no new
+  primitive. Seen working in the phone-sized render (a real ▶ beside
+  "Edit"), and its two tests check it points the right way and leaves its
+  corners clear.
 
 ### 3. "The benchmark scrolling seems like it's at 60fps or something, it's
 ### way less smooth than me scrolling manually"