SP: LargeAlloc return CBChunk & chase consequences

Even if we opt not to bound these pointers internally (if they aren't headed out
to the user program or we later derive bounded pointers), they should still be
annotated as something other than CBArena, ensuring that we do not attempt to
use them for general amplification.

src/mem/freelist.h

@@ -325,7 +325,8 @@ namespace snmalloc
      * Start building a new free list.
      * Provide pointer to the slab to initialise the system.
      */
-    void open(CapPtr<void, CBArena> p)
+    template<capptr_bounds B> // TODO: CBChunk-only
+    void open(CapPtr<void, B> p)
     {
       SNMALLOC_ASSERT(empty());
       for (size_t i = 0; i < LENGTH; i++)