Explain the terminal chart colours by the terminal theme

The docstring still justified the ANSI colours by the old one-hue web ramp.
The web now draws from the categorical --chart-1…8; the terminal keeps named
ANSI colours so it follows the user's terminal theme.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013GAeg4Jo6xjZzjwy9QDWMQ

backend/fluksio/tui/chart.py

@@ -6,10 +6,11 @@ chart. Nothing is imported for this: a plotting library would be two
 dependencies on a package whose every dependency is argued for in
 `pyproject.toml`, for something a screenful of arithmetic does.
 
-The colours are deliberately not the web's. `--chart-1…5` step one brand hue
-by lightness, which is right against a designed surface and unreadable in a
-terminal — five steps of one hue collapse to one colour on a 16-colour tty.
-Here identity is carried by hue, and the legend names every series anyway.
+The colours are deliberately not the web's. The web draws from the categorical
+`--chart-1…8` palette (DESIGN-GUIDELINES.md, "Chart palette"), fixed values
+chosen against its own surfaces. A terminal has a theme of its own, so these
+are named ANSI colours that the user's terminal maps to its scheme, light or
+dark. Identity is carried by hue, and the legend names every series anyway.
 """
 
 from __future__ import annotations