gh-153946: Add API to defer automatic garbage collection

[pablogsal]

• Issue: Add a private API to defer automatic garbage collection #153946

[pablogsal]

I ran this a few times on release builds pinned to one CPU, comparing aeedae8 with this branch. These timings are the median of the medians from three independent runs, with 15-20 samples and 10 iterations per sample.

Workload	main	This PR	Change	Automatic collections
Parse eight large stdlib files into AST objects	96.22 ms	78.20 ms	18.7% faster	8 per batch -> 0
marshal.loads(), 50,000 records	12.73 ms	10.49 ms	17.6% faster	1 per call -> 0

I also ran perf stat -r 5 on the same CPython workloads. The AST case went from 26.25B to 24.14B instructions, 8.1% fewer, and task clock dropped 18.5%. The marshal case went from 4.25B to 3.57B instructions, 15.9% fewer, and task clock dropped 17.8%.

I tried scheduling a collection directly from _PyGC_ResumeAutomaticCollection() as well. That made AST conversion about 3% slower than main, so this version keeps counting and waits for the next normal scheduling point. I think this is the right behavior here.

[maurycy]

Do we need a similar check before this?

cpython/Python/gc.c

Lines 1486 to 1497 in fe0cc2b

	/* update collection and allocation counters */
	if (generation+1 < NUM_GENERATIONS) {
	gcstate->generations[generation+1].count += 1;
	}
	for (i = 0; i <= generation; i++) {
	gcstate->generations[i].count = 0;
	}
	/* merge younger generations with one we are currently collecting */
	for (i = 0; i < generation; i++) {
	gc_list_merge(GEN_HEAD(gcstate, i), GEN_HEAD(gcstate, generation));
	}

This is a very shy question, so please treat it with a huge grain of salt. My hunch is that GC callbacks may still cause race, since they're just regular Python code and may release GIL:

cpython/Python/gc.c

Lines 1467 to 1472 in fe0cc2b

	GC_STAT_ADD(generation, collections, 1);
	struct gc_generation_stats stats = { 0 };
	if (reason != _Py_GC_REASON_SHUTDOWN) {
	invoke_gc_callback(tstate, "start", generation, &stats);
	}

I interrogated Claude Opus to show me what is the actual place for invoking and returning from the callbacks, and that's how I arrived at 1486.