Remove constant_references from CoverageTracker

[ngan]

Summary

• remove constant_references from FastCov::CoverageTracker and the native FastCov::Coverage implementation
• delete the old Prism-based constant extractor and the benchmark/docs/specs that depended on it
• keep coverage focused on line events, allocation tracing, and dynamic const_get tracking only

Testing

• bundle exec rake compile
• bundle exec rspec --fail-fast spec/lib/fast_cov/cache_spec.rb spec/lib/fast_cov/coverage/caching_spec.rb spec/lib/fast_cov/coverage/line_coverage_spec.rb
• bundle exec rspec --fail-fast

Notes

This PR intentionally removes the existing constant-reference expansion path without adding StaticMap yet. The replacement static analysis API can be iterated on separately.