docs(showcase): rebuild Reva README and docs

[xt0n1-t3ch]

Summary

• Rebuilt the README from scratch around the real Reva logo, plain-English product narrative, and mobile-friendly showcase flow.
• Added clean live app screenshots plus polished marketing-card versions under docs/assets/showcase/.
• Rewrote the documentation set for interview/showcase readiness, including product guide, architecture, pipeline, demo script, packaging, interview cheatsheet, code tour, stack, model landscape, and domain context.
• Removed the old generated README banner that rendered poorly on mobile.

Root cause

The prior README was technically useful but not showcase-ready on GitHub mobile: the hero image did not use the app logo, capability tables were dense, and Mermaid diagrams rendered as raw code in the mobile view Tony reviewed.

Changes

• README.md: new app-logo header, concise product narrative, marketing screenshot showcase, non-programmer capability table, quick start, packaging summary, and docs index.
• docs/product-guide.md: new plain-English workflow guide for non-programmers and interviewers.
• docs/index.md: rebuilt documentation map and constraints without Mermaid blocks.
• docs/architecture.md: rewrote boundaries, runtime flow, contracts, and scaling path.
• docs/ai-pipeline.md: rewrote deterministic pipeline, learned mapping, optional model assist, and assistant tool loop.
• docs/demo-script.md: rewrote interview walkthrough and talk track.
• docs/packaging.md: clarified development, packaged mode, runtime data, and validation.
• docs/learn/*.md: rewrote interview cheatsheet, code tour, tech stack, and model landscape.
• docs/research/reinsurance-landscape.md: rewrote domain context in product-friendly language.
• docs/assets/reva-logo-*.png: copied the real app logo variants for README rendering.
• docs/assets/showcase/*.png: added clean live app screenshots and marketing-card derivatives.
• docs/assets/reva-banner.png: removed stale generated banner.

Proof

• pnpm build in web/ passed: Next.js 16.2.9 compiled, TypeScript completed, and 11 static pages generated.
• git diff --check passed.
• Markdown image validation script passed: 12 markdown files checked, all local image links exist, no stale reva-banner.png references, and no raw Mermaid code blocks remain.
• Live app screenshots were captured from http://localhost:3000 with onboarding disabled and API data from http://localhost:5158.

Acceptance

• README uses the same logo as the app: docs/assets/reva-logo-dark.png and docs/assets/reva-logo-light.png.
• README includes a polished Showcase section using real app screenshots.
• Documentation is rewritten in professional, easy-to-understand language for non-programmers and interviews.
• Mobile rendering no longer relies on Mermaid blocks or the old generated banner.
• No application behavior changed.

Closes #57