Refactor coverage orchestration around CoverageMap

[ngan]

Summary

• replace the global FastCov singleton/configuration flow with instance-based FastCov::CoverageMap
• make ignored_paths plural and flexible while keeping tracker ownership/config on the coverage map instance
• remove native allocation tracing and the old cache/configuration/coverage tracker plumbing so coverage is line-based plus optional extra trackers

Testing

• bundle exec rake compile
• bundle exec rspec --fail-fast