added physical memory manager

docs/memory-map.md

@@ -131,12 +131,12 @@ kernel with a **device-tree blob**; the AArch64 entry code will parse its
 array. The kernel's memory code — the frame allocator and everything above it —
 never knows the difference.
 
-## What's next (not done here)
+## What's next
 
-This is plumbing plus classification only. Still to come:
+This page is plumbing plus classification only. The map's first consumer, the
+**physical frame allocator**, is built directly on the `usable` regions here —
+see [frame-allocator.md](frame-allocator.md). Still to come after that:
 
-- A **physical frame allocator** that consumes `usable` regions and hands out
-  4 KiB frames — the foundation everything else stands on.
 - Reclaiming `reclaimable` regions, and carefully freeing `reserved` `loader_data`
   (kernel image, these buffers) once the kernel is done reading them.
 - Paging / the kernel's own page tables, then a heap.