Add nested category support for module and project menus

[Copilot]

Module and project menus currently display as flat lists, limiting organization for large example sets. This adds support for nested categories using slash-delimited paths (e.g., Functions/Math, Effects/Audio).

Changes

• Type system: Added optional category field to ProjectMetadata

• Category tree builder (categoryTree.ts): Converts flat category lists into navigable tree structure

  • Parses category: "A/B/C" into nested nodes
  • Alphabetically sorts categories and items at each level
  • Defaults missing categories to "Uncategorized"

• Menu navigation: Refactored moduleCategoriesMenu and projectMenu to support nested traversal

  • Added categoryPath parameter for navigation (e.g., "Functions/Math")
  • Each node shows subcategories (as submenus) and items (as actions)
  • Backward compatible with flat categories

• Metadata updates: Organized ~110 modules and 14 projects into nested hierarchies

  • Modules: Functions/Buffer, I/O/MIDI, Sequencers/Break Beats
  • Projects: Music/MIDI, Graphics/Post-Processing, Machine Learning

Example Usage

// Before: Flat categories only
{ category: 'Random' }
{ category: 'Buffer' }

// After: Nested categories supported
{ category: 'Functions/Random' }
{ category: 'Functions/Buffer' }
{ category: 'Effects/Audio' }

// Menu navigation builds tree automatically
// Root → "Functions" → "Random" → "XORShift"

All 458 tests pass including 26 new tests for tree building and nested menu generation.

Original prompt

Please do this todo: docs/todos/126-nested-example-menus.md

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