Add initial SignalGrid visual asset kit

[DanFashauer]

Motivation

• Provide an initial set of scalable SVG brand assets (primary logo, compact mark, README hero, social preview, LinkedIn teaser) to align repo visuals with docs/BRAND_SYSTEM.md and startup/branding/brand.md.
• Supply a lightweight usage guide and export guidance while reinforcing claim boundaries so visuals are used as conceptual/demo assets rather than production claims.

Description

• Add public/signalgrid-logo.svg as the primary mark + wordmark using warm charcoal, off-white, and muted teal and designed to scale to favicon sizes.
• Add public/signalgrid-logo-mark.svg as an icon-only monogram suitable for favicons/avatars and small placements.
• Add docs/assets/github-social-preview.svg, docs/assets/readme-hero.svg, and docs/assets/linkedin-teaser.svg as restrained SVGs for social/README/teaser usage (1200×630 social preview included).
• Add docs/VISUAL_ASSETS.md documenting asset purposes, where to use them, PNG export guidance, and a reminder to follow docs/CLAIM_BOUNDARIES.md, and lightly update README.md to include the new logo and hero.

Testing

• Ran git diff --check and it completed without failures.
• Ran npm run lint (ESLint) and it completed successfully with a non-blocking npm environment warning.

---

Codex Task