Previously every 16 KiB HeapBlock was its own posix_memalign /
mach_vm_map / VirtualAlloc, which churned VMAs and made the kernel's
vm_area_struct list balloon for any non-trivial heap.
Carve slots out of 2 MiB chunks instead. The kernel now sees one
mmap per 128 blocks. Chunks are owned exclusively by a single
BlockAllocator and are never released back to the OS or shared
across allocators -- that's how we keep the heap's VM permanently
type-isolated, where a virtual address used for a cell of type T
is never reused for any other type. We don't bother tracking chunk
bases for teardown: the destructor leaks them by design.
Per-block memory return is preserved: deallocate_block still calls
MADV_FREE_REUSABLE / MADV_FREE / MADV_DONTNEED / DiscardVirtualMemory
so the kernel can reclaim physical pages under pressure.
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