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.

library/runtime/system-call.zig

@@ -6,8 +6,8 @@
 //! Note argument #3 goes in **r10, not rcx** — rcx is unavailable across the
 //! instruction, so the kernel reads the 4th argument from r10.
 
-const danos = @import("danos");
-const SystemCall = danos.SystemCall;
+const system = @import("system");
+const SystemCall = system.SystemCall;
 
 pub inline fn systemCall0(n: SystemCall) usize {
     return asm volatile ("syscall"