Create ratio-based spacing scale skill

[vnys]

Generate a spacing scale from a base unit and ratio, in the same spirit as the type-scale skill.

[vnys]

Source pointers (in equinor/ids-meetup-oslo-26):

• eds-tokens-reworked/src/spacing.ts — the ratio ladder itself (ladderIndex/ladderValue, golden-ratio derivation, snapping), PROPORTION_OFFSET (squished/squared/stretched as ladder shifts, not new values), and the header comment recording the anisotropic-density seam: horizontal and vertical tokens are split with identical values because density was designed to one day tighten vertically more than horizontally (compact = more table rows, not narrower tables). That recovered intent is the skill's best story.
• eds-tokens-reworked/DECISIONS.md — the gap-snap decision (0.5px → 2px after blurry-icon reports; 32 pinned deviations, the harness taught to expect them) and the spacing-axes resolution (keep the seam, don't implement the divergence).
• eds-contracts/src/resolve.ts → opticalGeometry — how the spacing ladder composes with typography: padding = inset − halfLeading(label), so control heights emerge instead of being authored.
• The naming lesson (from the 2026-08-28 discussion): a ratio system fails when token NAMES try to encode every coordinate (spacing-medium-vertical-squished-optical-…); it works when names stay human vocabulary and the coordinates are computed by contracts/recipes. The skill should teach that split.