Rename the shared contract module danos -> system; QEMU logs to /var/log/system

The shared kernel<->user ABI contract (BootInformation, the SystemCall numbers,
DeviceDescriptor, page_size, ...) is now the `system` module at
system/system.zig, following the convention that a directory's root file takes
the directory's name.

One overlap to note: the runtime's syscall wrappers are already `runtime.system`,
so the single file that uses both the contract and those wrappers
(library/runtime/heap.zig) aliases the wrappers locally as `system_calls`. The
two are distinct (top-level `system` vs `runtime.system`); everywhere else the
contract is just `system`.

Also: the QEMU run's serial capture now lands in the FHS log location,
zig-out/var/log/system/serial0-<timestamp>.log — a stand-in for the kernel's own
logging system, which will eventually write there itself.

Suite 35/35 plus host tests green.

system/services/vfs/vfs.zig

@@ -1,4 +1,4 @@
-//! /sbin/vfs — the user-space VFS server. Shipped in the initial_ramdisk, spawned as a
+//! system/services/vfs — the user-space VFS server. Shipped in the initial_ramdisk, spawned as a
 //! ring-3 process, and reached by every other process through IPC (the `runtime`
 //! file API marshals open/read/write/stat/close into calls to this server's
 //! endpoint, published under the well-known `vfs` service id).