Record the appearance switch as a component, not just a mechanism

[Babissimo]

Stacked on #22, which documents the palette pair and the three-versus-two-state
divergence. This adds the part that was still missing: the control itself.

§3 described what the theme switch does to a surface and never described the
thing a person clicks. Read cold, the guide told you which half of the palette
you would get and left the control to be invented per surface. That is exactly
what happened: Tower Finder grew a two-state chip of its own before anyone
noticed dash already had a three-state radiogroup
(offworldlabs/tower-finder-service#35).

So §6 gains an entry for .theme-switch, and tokens.css gains its rules.

The styling is the easy half and not the point. Six declarations. What is
worth writing down is the markup contract underneath: the buttons hold a glyph
and no text, so without an aria-label and a matching title a screen reader
meets three unlabelled radios; and choosing role="radio" over three toggle
buttons obliges the radiogroup keyboard behaviour, one tab stop with the arrows
moving selection and focus together. Both are invisible in a screenshot and
neither survives being copied by eye, so both sit beside the CSS rather than
inside it.

Also recorded:

• The switch wants a surface under it, not a canvas. Its ground is the sunk
  tier, and on the light half of these surfaces that is the canvas value (§3), so
  one dropped straight onto a page reads as three loose glyphs with only its
  hairline bounding it.
• The map's control is a different object and predates the others: a
  role="menuitem" button in the toolbar overflow menu, two states, none of the
  radiogroup behaviour. §10's divergence bullet now says the controls differ as
  much as the behaviour does.
• Tower Finder, as a fourth surface running the pair from outside
  retina-server. Deliberately with no column in the tables and no voice in §1:
  it is dash's look on another host, and a seventh column would imply a seventh
  personality it does not have.

No change to any palette value.

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