Add dev-only spacing scale lab for A/B testing.

[wailingwombat]

Compare grid4, Fibonacci, and modular unified scales on localhost:3000 via a floating switcher without affecting production builds.

Summary

• Adds an experimental dev-only Scale switcher on http://localhost:3000 to compare three unified spacing systems: 4px grid (default), Fibonacci, and Modular (~1.5×).
• Scale presets drive spacing, border radius, typography, and icon sizes via scale-presets.ts and dynamic theme builders (getStyledTheme / getHonorableTheme).
• Selection persists in localStorage (plural-prototype-scale). Production builds always use grid4 — the toggle is gated on import.meta.env.DEV.

Test plan

• git checkout anna/spacing-scale-lab && cd assets && yarn start:cd
• Open http://localhost:3000 — confirm Scale panel appears bottom-right
• Switch between 4px grid / Fibonacci / Modular and verify tables, forms, buttons, and icons reflow
• Refresh page — selected scale should persist
• Confirm production build (yarn build) does not include the toggle (grid4 only)

Notes

Experimental branch for design review

Checklist

• I have added a meaningful title and summary to convey the impact of this PR to a user.
• If required, I have updated the Plural documentation accordingly.
• I have added tests to cover my changes.
• I have deployed the agent to a test environment and verified that it works as expected (required only when changing agent code).

Plural Flow: console