v0.2.65: Readable secondary text, and :theme that actually repaints

Fixed

• Secondary text is now readable in every theme. The palette bridge mapped the render-time colours onto
  the wrong rungs of each theme's text scale: prose used the text_dim step and faint text used text_ghost,
  while text_muted (the step meant for prose) went unused entirely.

  On the SuperQode theme secondary text measured 4.35:1 against the background, below the 4.5:1 WCAG
  requires for body text, and faint text measured 2.72:1. On Dracula the same bug put faint text at 1.56:1
  — effectively invisible.

  Each rung now shifts up one, fixing every theme at once. The brand gradient is untouched, and a test
  asserts it verbatim so contrast work can never dilute the identity.

• :theme looked like it did nothing. It correctly replaced all 21 palette colours, but refresh()
  cannot recolour text whose styles were resolved when it was written, so the screen kept the old palette. The
  home screen is now rebuilt from source and a theme change is visible immediately. A conversation is never
  cleared for a cosmetic command: when a transcript is present, the command explains that earlier output keeps
  its original colours and that :home repaints in full.

Added

• scripts/show_theme_contrast.py prints a theme's text colours as real terminal swatches with their
  measured contrast ratio, because a colour change is hard to judge from a diff.