Docs: expand WVO brand and repository overview

[Artsen]

Summary

• Expands docs/brand-system.md into the canonical WVO product, visual, interaction, accessibility, and writing system.
• Expands the root README into a fuller product and repository overview with workflow, architecture, processing, generated assets, local runtime, storage, screenshots, and documentation map.
• Corrects the user guide comparison-mode wording to match the current Grid, Wipe, and A/B implementation.

Scope

• Documentation-only product and repository explanation work.
• No application behavior changes.
• No production code changes.
• No package version or runtime configuration changes.

Verification

• npm ci
• npm run format:check
• npm run lint
• npm run typecheck
• npm run test:run — 388 passing, 1 skipped
• npm run build
• npm run check
• git diff --check

Notes

The README intentionally summarizes the deeper docs instead of duplicating the full architecture, API, testing, configuration, and user-guide documents.