feat: add landing page with interactive demo and roadmap

[ditvor]

Summary

• Single-page marketing site at landing/index.html
• Hero section, problem/solution, how-it-works, roadmap (Phases 1–3)
• Interactive IDE mockup demo using mock data from Phase 1 test fixtures (PROJ-4521, AUTH-88, PERF-12)
• No backend required — fully static, no real API calls

Test plan

• Open landing/index.html in a browser
• Click each demo ticket pill and confirm the mock output animates in
• Verify the roadmap shows Phase 1 as complete, Phase 2 as upcoming, Phase 3 as planned
• Check responsive layout on mobile width

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