Fix encoding issues and remove temporary documentation files

1. Fix encoding in ModifiedGradientDescentOptimizer.cs:
   - Replaced Unicode symbols (η, ∇, ⊗, ℓ) with ASCII equivalents
   - Changed η to 'eta', ∇ to 'gradient', ⊗ to 'outer-product'
   - Updated all documentation to use only ASCII characters
   - Follows codebase standard of using plain English, not Unicode math symbols

2. Remove temporary documentation files:
   - Deleted COMPREHENSIVE_PAPER_VERIFICATION.md
   - Deleted PAPER_VERIFICATION_FINDINGS.md
   - Deleted nested_learning_paper.txt
   - Deleted MODIFIED_GD_INTEGRATION_PLAN.md
   - Deleted NESTED_LEARNING_IMPLEMENTATION_SUMMARY.md

These verification docs were helpful during development but should not
be checked into the repository.

Author: claude