Fix GC stack scanning bugs: memcpy direction in gc_save_context and volatile stopping_world

[Copilot]

Intermittent segfaults in multithreaded workloads (reproduced via hxcoro tests, ~40% failure rate on multi-core, 0% on single core). Crash is always in hl_to_virtual accessing a GC-collected object on a worker thread.

Changes

• src/gc.c — Fix gc_save_context memcpy source address: The memcpy copied from prev_stack (caller's variable address) upward, capturing data above the gap that's already covered by the main gc_mark_stack(stack_cur, stack_top) scan. The actual callee-saved register spill area lives between stack_cur and prev_stack — that's what needs preserving in extra_stack_data before it gets overwritten by subsequent calls (e.g. hl_mutex_acquire). Also added a size < 0 guard.

// Before: copies wrong region (above the gap, redundant with main stack scan)
memcpy(t->extra_stack_data, prev_stack, size*sizeof(void*));

// After: copies the actual gap (callee-saved register area)
memcpy(t->extra_stack_data, stack_cur, size*sizeof(void*));

• src/hl.h — Make stopping_world volatile: This flag is written by the GC thread and read by worker threads in hl_blocking(false) and hl_gc_safepoint(). Without volatile, the compiler is free to cache the value.

Known remaining issue

There is a deeper race in hl_blocking(false): a thread can transition gc_blocking from 1→0 after gc_stop_world has already passed it in its sequential check. In that window the thread runs JIT code with GC roots only in scratch registers, invisible to the stale gc_regs/stack_cur. The proposed fix is to spin-wait in hl_blocking(false) while stopping_world is true before allowing gc_blocking to reach 0, but this is not yet implemented in this PR.

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[Simn]

I have pushed the volatile stopping_world change to master because that already came up in #899 and was greenlighted there.

[Simn]

@ncannasse Please check this part!

[ncannasse]

I don't think it's correct.

[Simn]

Alright, I'll stop with the AI stuff for HL because it doesn't appear to actually help here. Thanks for checking!