Function alloc_owned_mutable_ptr 

pub fn alloc_owned_mutable_ptr(initial: u64) -> *mut u64

Allocate a fresh OwnedMutable cell holding a Ptr payload.

The cell stores the raw 8-byte heap-pointer bit pattern (a NaN-boxed ValueWord carrying an Arc<HeapValue> share or an owned heap pointer). The interior bits are released through [release_raw_value_bits] inside drop_owned_mutable_capture before the box itself is reclaimed, mirroring the existing heap_capture_mask semantics for immutable Ptr captures.