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/testing.md

@@ -8,7 +8,7 @@ without a human staring at the screen.
 There are two layers:
 
 - **Host unit tests** (`zig build test`) — for pure, platform-independent logic in
-  the shared `danos` module (the handoff layout in `system/danos.zig`). These compile
+  the shared `system` module (the handoff layout in `system/system.zig`). These compile
   for the host and run natively.
 - **QEMU integration tests** (`python3 test/qemu_test.py`) — boot the real kernel
   and check its behaviour. This is the interesting part.