Describe GC for free-threaded build in the GC design doc

[colesbury]

This updates the garbage collector design page to describe the implementation of the GC for Python 3.13's free-threaded build.

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📚 Documentation preview 📚: https://cpython-devguide--1263.org.readthedocs.build/

[colesbury]

I think there's still some more I should add to the document, but I wanted to first get feedback on the general approach of mixing the descriptions of the two designs.

[ezio-melotti]

This could use a list to make the difference between the two builds stand out more, and to mirror the two sections below. Something like:

Suggested change

	CPython contains two GC implementations starting in version 3.13. One implementation
	is used in the default build and relies on the global interpreter lock for
	thread-safety. The other implementation is used in the free-threaded build. Both
	implementations use the same basic algorithms, but operate on different data
	structures.
	Starting in version 3.13, CPython contains two GC implementations starting in version 3.13:
	* the default build implementation relies on the global interpreter lock for
	thread-safety;
	* the free-threaded build ...
	Both implementations use the same basic algorithms, but operate on different data
	structures.

Here you could also add link to :term:s like GC and GIL using intersphinx.

[colesbury]

I added a section at the end to summarize the differences between the two implementations. I was hesitant to add it here because we introduce a number of the concepts later on in the document.

[ezio-melotti]

Do each --+ mark one bit? If so, ob_tid, ob_ref_shared and *ob_type are 2 bytes (16 bits), but this fields don't seem to match the description, since ob_gc_bits takes ~4 bits and ob_ref_local 6.

[colesbury]

No -- each row is 64-bits (on 64-bit platforms), but otherwise the fields pad, ob_mutex, ob_gc_bits, and ob_ref_local are not to scale.

I'm not sure what to do about this. It'd be nice if the diagram were to scale, but I think that would require the diagram to be very wide -- ob_gc_bits is only 1 byte, but 10 characters.

[ezio-melotti]

It might be enough to just add a note after the diagram saying that e.g. "Note that pad, ob_mutex, ob_gc_bits, and ob_ref_local are not to scale and take X, Y, Z, W bytes respectively."

Another option, if those 4 values are related, could be to use a single name (like "free-threaded info bytes") and then break that down afterwards, either in text or with a separate "zoomed in" diagram.

[ambv]

Let's get this in and iterate on the live document.