Grow the guest heap through the store, not around it

[Arshia001]

Needs the paired wasmer change: wasmerio/wasmer#6887 (stacked on #6886). It will not build against wasmer main until those land.

Why

GuestHeap grew guest linear memory through a cloned VMSharedMemory, obtained from a SharedMemory::vm_shared_memory() accessor that existed for this one caller. Growing a memory outside the store that owns it is not an operation the wasmer API should have, and it is being dropped. wasmer instead grew a way to hand the store to code that cannot be passed one.

The bind this is in: the allocation that needs the growth arrives with no store in sight. V8 calls the array-buffer allocator from inside its own frames, several levels below the N-API import that entered it, and no Rust reference survives that trip.

What changed

The import lends its store across the boundary — StoreMut::parked — and the heap picks it back up with Store::with_current, then grows through the ordinary Memory::grow. Lending happens in with_callback_state, which already wraps every bridge call that can run JS, plus the handful of import handlers that allocate directly (via the new alloc_guest helper).

Fallout worth knowing about:

• The shared/owned split is gone. Any heap grows when a store is lent and falls back to its arena when none is — which is what a V8 background thread will find. So every heap is now pre-funded and topped up from import boundaries, not just the non-shared ones.
• Handles are store-scoped. WASIX worker threads share one linear memory but each has its own store, so the heap keeps one handle per store that reaches it and grows through the one belonging to the store being lent. get_or_create registers the caller's.
• The style check reads MemoryStyle off the engine's tunables instead of the VM memory, so the file no longer reaches into wasmer::sys::vm at all.
• Pre-funding is best effort. An unfunded heap is slower, not broken, since any allocation made with a store lent can claim for itself.

callback.rs moves with it. Its callback context held a raw pointer to the installing frame's FunctionEnvMut — which is precisely the borrow the import now parks, and reaching around a park would defeat the point of it. It holds a store-free FunctionEnv handle instead and picks the store up the same way the heap does. That removes the last raw-pointer smuggling from the file.

rust-toolchain.toml goes to 1.95, which wasmer's MSRV now requires.

Testing

40/40 lib tests. New ones cover: an allocation served from the arena with no store lent at all; the same allocation past the arena failing rather than growing the guest's memory behind its back; the same request succeeding once a store is lent; and a store that never registered a handle being unable to grow the memory.