gh-142516: Forward-port generational GC

[zanieb]

This patch is was created by taking the following two branches as references

• https://github.com/sergey-miryanov/cpython/tree/feat/gc-3g
• https://github.com/nascheme/cpython/tree/gc-gen-3.14

and removing the runtime toggle behavior included in their implementations then doing multiple comparative passes to the 3.13 implementation and changes to the GC since 3.13.

This process was performed using an LLM and is intended as a starting point, not a final patch — as I do not have sufficient CPython GC expertise to audit the work.

See https://discuss.python.org/t/reverting-the-incremental-gc-in-python-3-14-and-3-15/107014 for broader context.

The 3.14 backport can be found at #148694

• Issue: Observed memory leak in ssl library: Python 3.14 GC issue #142516

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📚 Documentation preview 📚: https://cpython-previews--148687.org.readthedocs.build/

[sergey-miryanov]

I've cherry-picked the commit to this branch: #148720. I added a Co-Authored-By trailer to the cherry-picked commits, and I hope GitHub will preserve the original authorship when the PR is merged.

Thanks!

[zanieb]

Closing in favor of #148746 / #148720