docs(brand): describe the map as a themed surface on dash's system

[Babissimo]

map.retina.fm has been restyled on dash.retina.fm's design system, and the brand guide still described the surface that replaced. This brings docs/brand/brand-guide.md and docs/brand/tokens.css back in step with it.

Land this after the retina-server stack

The map's restyle is a four-PR stack, open and unmerged at the time of writing:

• Extract the map's colours into one palette module retina-server#343 feat/map-1-palette, the palette module
• Restyle the map's chrome on dash's design system retina-server#345 feat/map-2-chrome, the chrome on dash's system
• Separate the map's colours instead of leaving them merely visible retina-server#346 feat/map-3-contrast, the measured data colours
• Make the palette a theme, and offer light rather than impose it retina-server#347 feat/map-4-theme, the light theme

Everything here is written against the tip of that stack. It should not merge before the stack does, or the guide will describe a surface that is not yet live. If the stack changes shape in review, this needs re-reading against what actually lands.

What changed in the guide

The map is no longer dark-only, no longer without custom properties, and no longer without focus or reduced-motion handling, so every blanket claim resting on those has been narrowed to the surfaces it still describes rather than rewritten as a before-and-after.

• §1, §2, §3 record the map as themed: dark is the default and unchanged, light is opt-in and takes dash's values. The mechanism is recorded once, in §3, because it is the transferable part: the base selector carries dark and the attribute carries light, so the default cannot flash the other theme before first paint.
• §3 gains the reason the map needs a sunk tier that the light surfaces do without, a Tint (hover) row for --accent-tint, and a corrected border value (.12 to .14, drifted when the hardcoded hex was consolidated into tokens).
• §3 and §8: the data palette was rechosen by measurement, against a contrast floor and a CIEDE2000 separation between marks. Truth now wears no hue in either theme, so the old teal/cyan description was not merely stale but contradicted the surface.
• §5, §6, §10 drop the map from the no-prefers-reduced-motion and no-:focus-visible gaps, and the shared-control and token-drift bullets narrow to dash. The map and dash are now the only pair in the estate sharing a vocabulary.
• §6's map entries went stale wholesale: the gradient header, the two ad-hoc search boxes, the near-invisible empty state, the backdrop-blurred panels and the unstyled Leaflet controls are each now one thing rather than several.

tokens.css gains --accent-tint and a .map[data-theme=\"light\"] block. The attribute variant mirrors the live selector exactly and stays keyed on .map, so it fits the one-class-per-surface structure rather than fighting it. --data-adsb is dropped: the ground-truth dot and truth are one colour now, and the pair is replaced by --data-truth / --data-truth-dark.

Two things worth flagging

preview.html carried two captions this PR deletes from the guide. It annotated the map with "Two search boxes, one job", a defect the map stack fixed by giving the surface one form-control rule in its token layer, and led with "the dark operational picture" when dark is now the default rather than the whole surface. Both are rewritten.

No light-theme section was added. It would be dash's values rendered a second time, since the map's light theme takes them wholesale. The two console tags now say which mode each one is, and the guide carries the pairing in full.

§5's radius numbers are now recorded. The map's radius scale is dash's: 8px default, 4px small, 12px large. It reached the guide late because the three geometry tokens were briefly declared only inside the light-theme block, so the default theme rendered none of them; that is fixed on feat/map-4-theme and verified in the built bundle.

Conflict risk

Low, now. The worktree-docs+owl-os-brand-surface work merged as #20 before this branch was cut, so this is based on a main that already contains the owl-os column and the expanded §6. It was that expansion that put most of the stale map claims in §6 here in the first place.

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