fix: allow forward exports from package entrypoints

[artiphishle]

Closes #50

Summary

• keep ankhorage/no-forward-exports strict for ordinary implementation modules;
• allow forward exports from public package entrypoints declared by package.json main, types, and exports;
• map standard dist/... output targets back to src/... source entrypoints;
• share package metadata path resolution with ESLint profile detection;
• add integration coverage for non-index entrypoints and undeclared implementation files.

This fixes false positives such as src/root.ts in @ankhorage/studio without hardcoding Studio filenames or adding lint disables.