[Bug Report] GC Crash in Multithreaded Python Program Using @codon.convert Class + Python Object + ThreadPoolExecutor

### **Problem Description**
I encountered a GC crash (garbage collection segmentation fault) when running a multithreaded Python program using the Codon compiler. The crash occurs during concurrent execution with ThreadPoolExecutor and a @codon.convert decorated class.
### **Environment**
- OS: Debian GNU/Linux 12 (bookworm)
- Codon version: 0.19.3 && the latest version 0.19.6 build from source (develop codon branch)
- Python version: 3.11.10
### **Minimal Reproducible Code**
```
import codon
from concurrent.futures import ThreadPoolExecutor

@codon.convert
class PyMockCPUBusy:
__slots__ = ["compute_iters"]

def __init__(self, compute_iters: int):
self.compute_iters = compute_iters

@codon.jit
def __call__(self, obj):
count = 0
target = self.compute_iters
while count < target:
count += 1
return obj

def worker(tid: int, op: PyMockCPUBusy):
for i in range(20000):
op(None)

if __name__ == "__main__":
num_threads = 16
op = PyMockCPUBusy(100000)

with ThreadPoolExecutor(max_workers=num_threads) as ex:
futures = [ex.submit(worker, tid, op) for tid in range(num_threads)]
for f in futures:
f.result()
```
### **How to Run**
```
python3 test.py
```
### **Crash Frame**
```
Collecting from unknown thread
Aborted (core dumped)
#0 0x00007fff4a360f12 in GC_find_limit_with_bound () from /data00/home/leelee/.codon/lib/codon/libcodonrt.so
#1 0x00007fff4a360d30 in GC_init_linux_data_start () from /data00/home/leelee/.codon/lib/codon/libcodonrt.so
#2 0x00007fff4a35efef in GC_init () from /data00/home/leelee/.codon/lib/codon/libcodonrt.so
#3 0x00007fff4a362218 in GC_add_roots () from /data00/home/leelee/.codon/lib/codon/libcodonrt.so
#4 0x00007fff4d86afb6 in codon::BoehmGCJITLinkMemoryManager::allocate(llvm::jitlink::JITLinkDylib const*, llvm::jitlink::LinkGraph&, llvm::unique_function > >)>) ()
from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#5 0x00007fff522874c2 in llvm::jitlink::JITLinkerBase::linkPhase1(std::unique_ptr >) ()
from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#6 0x00007fff52263f58 in llvm::jitlink::link_ELF_x86_64(std::unique_ptr >, std::unique_ptr >) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#7 0x00007fff5222413a in llvm::jitlink::link_ELF(std::unique_ptr >, std::unique_ptr >) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#8 0x00007fff521ef75d in llvm::jitlink::link(std::unique_ptr >, std::unique_ptr >) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#9 0x00007fff50a31cc8 in llvm::orc::LinkGraphLinkingLayer::emit(std::unique_ptr >, std::unique_ptr >, std::unique_ptr >) ()
from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#10 0x00007fff50a81479 in llvm::orc::ObjectLinkingLayer::emit(std::unique_ptr >, std::unique_ptr >) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#11 0x00007fff50a81849 in llvm::orc::ObjectTransformLayer::emit(std::unique_ptr >, std::unique_ptr >) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#12 0x00007fff50ac76c3 in llvm::orc::IRCompileLayer::emit(std::unique_ptr >, llvm::orc::ThreadSafeModule) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#13 0x00007fff50ac7f47 in llvm::orc::IRTransformLayer::emit(std::unique_ptr >, llvm::orc::ThreadSafeModule) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#14 0x00007fff50ac7f47 in llvm::orc::IRTransformLayer::emit(std::unique_ptr >, llvm::orc::ThreadSafeModule) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#15 0x00007fff50a28537 in llvm::orc::BasicIRLayerMaterializationUnit::materialize(std::unique_ptr >) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#16 0x00007fff509f1a8b in llvm::orc::MaterializationTask::run() () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#17 0x00007fff509f1e41 in llvm::orc::ExecutionSession::dispatchOutstandingMUs() () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#18 0x00007fff509f2f3d in llvm::orc::ExecutionSession::OL_completeLookup(std::unique_ptr >, std::shared_ptr, std::function >, llvm::DenseMapInfo, llvm::detail::DenseMapPair > > > const&)>) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#19 0x00007fff509f3d33 in llvm::orc::InProgressFullLookupState::complete(std::unique_ptr >) ()
from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#20 0x00007fff509f6c93 in llvm::orc::ExecutionSession::OL_applyQueryPhase1(std::unique_ptr >, llvm::Error) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#21 0x00007fff509f73f2 in llvm::orc::ExecutionSession::lookup(llvm::orc::LookupKind, std::vector, std::allocator for more, q to quit, c to continue without paging--
air > > const&, llvm::orc::SymbolLookupSet, llvm::orc::SymbolState, llvm::unique_function, llvm::detail::DenseMapPair > >)>, std::function >, llvm::DenseMapInfo, llvm::detail::DenseMapPair > > > const&)>) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#22 0x00007fff509f7601 in llvm::orc::ExecutionSession::lookup(std::vector, std::allocator > > const&, llvm::orc::SymbolLookupSet, llvm::orc::LookupKind, llvm::orc::SymbolState, std::function >, llvm::DenseMapInfo, llvm::detail::DenseMapPair > > > const&)>) ()
from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#23 0x00007fff509f83ce in llvm::orc::ExecutionSession::lookup(std::vector, std::allocator > > const&, llvm::orc::SymbolStringPtr, llvm::orc::SymbolState) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#24 0x00007fff50a45b66 in llvm::orc::LLJIT::lookupLinkerMangled(llvm::orc::JITDylib&, llvm::orc::SymbolStringPtr) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#25 0x00007fff4d847421 in llvm::orc::LLJIT::lookup(llvm::orc::JITDylib&, llvm::StringRef) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#26 0x00007fff4d845686 in codon::jit::Engine::lookup(llvm::StringRef) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#27 0x00007fff4d84bdf1 in codon::jit::JIT::init(bool) () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
#28 0x00007fff4d84fb16 in jit_init () from /data00/home/leelee/.codon/lib/codon/libcodonc.so
```
### **Key Observations**
1. Crash only happens in **multithreaded mode** (ThreadPoolExecutor with ≥ 2 threads)
2. Crash should Involves:
- @codon.convert class
- @codon.jit method
- **A PyObject Type param needs to be passed in**
3. The code is CPU-bound and does not perform any heap allocation inside the JIT loop