added physical memory manager

src/root.zig

@@ -52,6 +52,10 @@ pub const MemoryKind = enum(u32) {
     acpi_tables,
     /// ACPI non-volatile storage: preserve across sleep, do not allocate.
     acpi_nvs,
+    /// Not backed by RAM: memory-mapped device registers or a reserved
+    /// address-space window (e.g. PCIe config space). Kept distinct from
+    /// `reserved` so RAM accounting doesn't count device address space.
+    mmio,
 };
 
 /// One contiguous span of physical memory. Because danos defines this layout