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/arch.md

@@ -83,7 +83,7 @@ There are really two independent questions, and it's worth not conflating them:
 
 The kernel entry point `_start` currently still lives in the generic `main.zig` as
 a thin trampoline into `kmain`. It's arch-adjacent (its calling convention is
-x86_64 [SysV](sysv.md), via the shared `danos.kernel_abi`), but it's three lines
+x86_64 [SysV](sysv.md), via the shared `system.kernel_abi`), but it's three lines
 and mostly generic, so it stays put for now. When AArch64 arrives — where entry means setting
 up a stack and reading a device-tree pointer from a register — the entry work will
 be substantial and per-arch, and *that* is when we extract an entry interface into