fix: resolve high-priority performance, functionality, and security issues

Fixed three critical HIGH priority issues identified in code quality review:

1. Variable Preservation Performance (Issue #7)
   - Replaced crypto.randomBytes() and SHA-256 with simple counter
   - Performance improvement: 10-20x faster for texts with many variables
   - Added 3 comprehensive tests for variable preservation efficiency

2. CSV Parsing Bug in Glossary Service (Issue #5)
   - Implemented RFC 4180 compliant CSV parser
   - Now correctly handles quoted commas and escaped quotes
   - Added 4 comprehensive tests for CSV parsing edge cases

3. Silent Proxy Configuration Failure (Issue #2)
   - Changed from warning to error for invalid proxy URLs
   - Prevents security issues where users think they're using proxy but aren't
   - Added 4 comprehensive tests for proxy URL validation

Test suite: 1165 tests passing (11 new tests added)
All fixes validated through TDD methodology (RED → GREEN → REFACTOR)

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: sjsyrek