Add README hero SVG and redesign homepage layout/styles

[DanFashauer]

Motivation

• Introduce a dedicated hero illustration and refresh the landing page to better explain SignalGrid's runtime decisioning and demo outcomes.
• Improve visual hierarchy and copy to support early-access conversion and clearer demo documentation.

Description

• Add a new hero illustration at public/readme-hero.svg that visualizes identity, device, and session inputs flowing into SignalGrid with Allow/Step-Up/Deny outcomes.
• Replace and rewrite src/app/page.tsx to a redesigned landing layout with updated colors, rounded containers, and a header that uses the new hero and logo assets.
• Add an outcomes array and map-driven UI for demo outcomes and a mapped step sequence for the feature flow, consolidating repeated markup into mapped lists.
• Update CTA copy and styles for the early-access flow and reorganize page sections (runtime access gap, how it works, demo outcomes, who it is for).

Testing

• No automated unit tests were added or modified in this change.
• Verified the application compiles and pages render locally in a dev build (TypeScript/JSX compilation succeeded).
• Manually inspected the homepage in the dev server to confirm the hero image, outcomes, and CTA render as expected.

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Codex Task