Add SignalGrid runtime decision architecture marketing infographic

[DanFashauer]

Motivation

• Provide a single, premium, marketing-ready SVG poster that explains SignalGrid’s runtime decision layer and where it sits in an enterprise orchestration flow.
• Replace a generic diagram with a cleaner, brand-aligned asset that communicates runtime inputs → decision → outcomes while staying within MVP/demo claim boundaries.

Description

• Add docs/assets/signalgrid-runtime-architecture.svg as an SVG-only marketing architecture poster containing the requested header, left-column runtime input cards (Identity, Device posture, Session context, Operational signals), central SignalGrid decision block with internal components, right-column outcome cards (Allow / Step-Up / Deny / Remediate / Record), bottom orchestration flow, strategy principles panel, and MVP-safe footer disclaimer.
• Apply brand-aligned styling using the repo palette (warm charcoal background, off-white typography, muted teal accent, and restrained state colors) and a minimal architectural grid for presentation-scale readability.
• Update docs/VISUAL_ASSETS.md to document the new asset with purpose, recommended use, export guidance, and a claim-boundary reminder.
• Keep the change SVG-only with no binary assets added and preserve copy consistent with docs/CLAIM_BOUNDARIES.md and docs/BRAND_SYSTEM.md guidance.

Testing

• Ran git diff --check which returned clean results and reported no whitespace/patch issues.
• Ran npm run lint (ESLint) which completed successfully for the modified files.
• No automated build (npm run build) or GitHub-hosted checks were run in this environment, so merge gating by CI should still be validated on GitHub before landing.

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Codex Task