docs: 📝 add token variable architecture ADR (typography & spacing) (#5161)

* docs(adr): 📝 add token variable architecture ADR (typography & spacing)

Defines the redefined EDS token system's variable structure — the primitive, mode-bearing mapping, and mode-free semantic layers plus the text-style layer — for typography, spacing, corner-radius, and density. Tokens Studio is the canonical source of truth; Figma variables sync from it. Colour and the code/output layer are out of scope (separate ADRs).

Status: Proposed. Refs #5139.

* docs(adr): 📝 correct type-scale naming rule and spacing tier range

Verified against Tokens Studio (canonical): the type scale uses a 100-step index (type-scale/{inter,equinor}/100…1100), not the rem-hundredths convention that applies to spacing; and the semantic spacing scale tops out at 3xl, not 4xl.

* docs(adr): reflow prose to one line per paragraph

* docs(adr): renumber token variable architecture ADR to 0007

* docs(adr): 📝 align 0007 ADR with template, unify header/color naming

Address review feedback on #5161:
- Add Decision Drivers and Options Considered; nest Consequences and
  Confirmation under Decision; rename References to Related, per
  0000-template.md.
- Unify the font-family axis onto the `header` spelling (was `heading`)
  and use it consistently.
- Spell out the full primitive to mapping to semantic reference chains
  for weight and family to disambiguate `weight/*` vs `font-weight/*`.
- Use US `color` throughout (Figma vars, CSS custom properties, TS).

Refs #5139.

Author: eddiman