Add comprehensive application description document

[Copilot]

Adds APP_DESCRIPTION.md - a 1,720-word platform-agnostic description covering the full application scope.

Content Structure

• Game mechanics: 9×9 gravity board, 5-in-a-row win condition, strategic depth analysis
• AI implementation: Three difficulty tiers (rule-based, weighted random, minimax with alpha-beta pruning), personality variants (Balanced, Aggressive, Defensive, Tricky)
• Player progression: Statistics tracking, leaderboard system, streak mechanics
• Architecture: Offline-first design, progressive enhancement, resilient API integration
• Testing: Unit, integration, and E2E coverage strategies
• Infrastructure: Cloud deployment, telemetry, declarative infrastructure
• Design patterns: SOLID principles, immutable state, vertical slice architecture

Document intentionally avoids framework/platform specifics to remain technology-agnostic while maintaining technical depth.

Original prompt

Give me a 1500 word description of this app that includes every detail we discussed without specifying any particular tech stack

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