feat: Add pipe obstacles with collision detection

[cyrusagent]

Summary

• ✅ Created Pipe class with position, width, height, and gap properties
• ✅ Implemented pipe spawning system that creates new pipes at regular intervals (every 2 seconds)
• ✅ Added horizontal movement for pipes moving right to left across the screen
• ✅ Created detailed pipe rendering with top and bottom pipe segments plus decorative caps
• ✅ Implemented precise AABB collision detection between bird and pipe rectangles
• ✅ Added pipe removal when they move off-screen for proper memory management
• ✅ Ensured proper gap spacing (120px) for bird to pass through - challenging but fair
• ✅ Added visual variety with different gap heights and randomized positions

Technical Implementation

• Pipe Class: Complete implementation with getBounds(), checkCollision(), render(), and lifecycle methods
• Collision System: Uses rectangle collision detection compatible with existing Bird collision system
• Spawning System: Time-based spawning every 2000ms with pipes starting off-screen
• Movement Physics: Delta-time based movement at 0.15 pixels/ms for smooth animation
• Memory Management: Automatic cleanup of off-screen pipes using isOffScreen() filtering
• Scoring Integration: Bird passing through pipes increments score with checkBirdPassed() logic
• Visual Design: Green pipes with caps matching classic Flappy Bird aesthetic

Test Results

All acceptance criteria verified and working:

• ✅ Pipes spawn at regular intervals and move across screen
• ✅ Collision detection triggers game over on bird-pipe contact
• ✅ Bird can successfully navigate through pipe gaps
• ✅ Score increases when bird passes through pipes
• ✅ Off-screen pipes are cleaned up properly
• ✅ Gap positioning provides challenging but fair gameplay
• ✅ Visual variety with randomized gap positions

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