Incremental GC verifier finds a reachable object left unmarked in gc_stress #424

The opt-in incremental collector left a reachable object unmarked in the gc stress test, and the verifier (HXCPP_GC_INCREMENTAL_VERIFY) aborted the run. It happened once, on the Haxe 4.3.7 / Linux 64 / incremental gc job:

https://github.com/openfl/libhxcpp/actions/runs/41359125947/job/125006688889

gc stress: 30 s, 8 threads, seed 2012663045
Incremental GC omitted reachable object 0x7f942007ef30 flags=12800801 epoch=23000000 rows=1 first=1 vtable=0x5597e36ca850
Aborted (core dumped) bin/Stress 30 8 2012663045

That is test/gc_stress built with -D HXCPP_GC_INCREMENTAL -D HXCPP_GC_INCREMENTAL_VERIFY, run as in test.yml. The abort came about 8.5 seconds into the 30 second run, not at exit.

Reading the header:

• The object is a container (0x00800000), one row long, and its line is marked.
• Its mark ID is 0x12, while the current one is 0x23, so the incremental cycle that just finished never reached it, though a stop-the-world walk from the roots did.

If the collector had gone on without the verifier, the object would have been swept while still reachable.

The seed is fixed, but with 8 threads the run isn't repeatable, and this job has passed on every recent main run, so it's intermittent. The run was on a branch whose only runtime change is at process exit (#423), which wasn't active yet at that point. I haven't reproduced it locally yet.