Improve constant mapping with ancestor expansion (default on)

[ngan]

Summary

This PR improves constant-reference mapping for shared concerns/mixins and enables the behavior by default.

Changes:

• Add ancestor expansion for constant references in the native extension.
• Add a dedicated cache bucket for ancestor file resolution (:const_ancestor_files).
• Add fixtures/spec coverage for dynamic include scenarios.
• Add benchmark scenarios for ancestor expansion (cold/warm cache).
• Change ancestor_references default from false to true.
• Update README docs for the new option/default and behavior.

Problem

We observed mapping gaps where a changed module/concern did not map to all consuming controller specs in CI. In eagerload/boot-heavy environments, a file can be loaded once and then reused, so line execution and simple file-load signals are insufficient to connect all impacted tests.

Solution

When constant_references is enabled, FastCov already resolves referenced constants to defining files. This PR adds optional ancestor expansion: if a resolved constant is a class/module, FastCov also resolves its ancestors (includes/extends/superclasses) and adds those source files to impacted output.

With this change, updates to shared mixins/concerns are much more likely to map to tests that reference consumers, even when the concern file itself is not re-executed during that worker's run.

Default Behavior

ancestor_references now defaults to true.

You can disable it explicitly:

config.use FastCov::CoverageTracker, ancestor_references: false

Benchmark (vs existing baseline)

From bin/benchmark:

• Constant resolution (cold cache): 0.398 ms (+1.7%)
• Constant resolution (warm cache): 0.006 ms (+20.5%)
• Constant resolution + ancestors (cold cache): 0.130 ms
• Constant resolution + ancestors (warm cache): 0.003 ms

Validation

• bundle exec rspec --fail-fast
• bin/benchmark --baseline (run before implementation)
• bin/benchmark (post-change compare)