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.

docs/paging.md

@@ -27,7 +27,7 @@ address to the low load address in its bootstrap tables and jumps in). The entir
 alongside a **physmap** — a straight window onto all of physical memory at
 `physmap_base + phys`. Wherever the kernel needs to touch a physical address (a
 page-table frame, an ACPI table, a device register), it adds that constant:
-`danos.physToVirt(phys)`. The layout constants live in `system/danos.zig`:
+`system.physToVirt(phys)`. The layout constants live in `system/system.zig`:
 
 | region | virtual base | PML4 slot |
 |--------|--------------|-----------|