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/devices/platform.zig

@@ -9,7 +9,7 @@
 //! compile-time choice.
 
 const std = @import("std");
-const danos = @import("danos");
+const system = @import("system");
 const device_model = @import("device-model.zig");
 const acpi = @import("acpi.zig");
 const power = @import("power.zig");
@@ -65,7 +65,7 @@ pub fn cpusDropped() usize {
 /// ACPI registers); pass the architecture implementation. Errors leave nothing to clean up
 /// beyond the tree's own allocations.
 pub fn discover(
-    boot_information: *const danos.BootInformation,
+    boot_information: *const system.BootInformation,
     allocator: std.mem.Allocator,
     hal: Hal,
 ) !DeviceTree {