Add dynamic-difficulty integration with glass-filling visualization

[carochacs]

Summary

Implements glass-filling difficulty visualization for song sections when the dynamic-difficulty plugin is installed.

Changes

• Add plugin detection for dynamic-difficulty plugin availability
• Implement glass-filling visualization showing difficulty percentage per section
• Add event listener for difficulty updates from dynamic-difficulty
• Gracefully degrade when dynamic-difficulty is not installed
• Export new utility functions for testing

How it works

The section map now:

1. Detects if the dynamic-difficulty plugin is loaded
2. Listens for difficulty updates via difficulty:sections-updated events
3. Renders a small glass indicator next to each section label showing fill percentage
4. Supports three glass sizes (small, medium, large) based on difficulty data
5. Degrades gracefully — displays sections normally if dynamic-difficulty isn't available

Acceptance criteria

• Only renders glass-filling when dynamic-difficulty is installed
• Glass fill level represents section difficulty
• Graceful fallback when plugin is unavailable
• Event-driven updates from dynamic-difficulty
• Exported functions for testing

Fixes #1

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