Optimize type equality fast paths

[bschnurr]

Summary

This optimizes isTypeSame by avoiding recursive comparisons when two types share the same immutable backing structures. The fast paths cover shared class type arguments, shared function parameter metadata, shared overload arrays, shared union subtype arrays, and shared TypeVar bound/constraint structures.

Details

The change keeps the existing structural comparison behavior for non-shared data, but skips recursive walks when reference identity proves the relevant backing structure is identical. This commonly applies to shallow-cloned or decorated types.

Validation

• npm run test:norebuild -- --runTestsByPath src/tests/typeEquality.test.ts --forceExit
• npm run build

Performance Notes

On a local full-project NumPy CPU profile, isTypeSame self time dropped from about 21.98s to 19.02s (-13.5%) after these fast paths. A sampled heap profile did not show the edited helper functions as allocation sites. Whole-program wall-clock timing remains noisy, so this PR is presented as a targeted hotspot reduction rather than a claimed end-to-end throughput win.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[rchiodo]

Approved via Review Center.