When a Wasm `throw` instruction executes, the `throw_ref` libcall was
cloning the GC ref (incrementing the refcount), but the catch handler
never decremented it. This caused every caught exception to leak,
leading to unbounded GC heap growth in throw/catch loops.

Two fixes:

1. Remove the unnecessary `clone_gc_ref()` in `throw_ref`. The
   `throw`/`throw_ref` instructions consume the exnref operand, so
   ownership transfers naturally to `pending_exception` without
   cloning.

2. In `create_catch_block`, emit a `drop_gc_ref` call for non-ref
   catches (`Catch::One`, `Catch::All`) after field extraction. These
   catches consume the exnref without passing it to the branch target,
   so the refcount must be decremented.

Also adds `Store::gc_heap_size()` / `StoreContext::gc_heap_size()`
accessors and a `throw_catch_many_times` integration test that throws
and catches 100K exceptions in a loop, asserting the GC heap stays
within a single 64 KiB page.