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/kernel/architecture/x86_64/apic.zig

@@ -9,7 +9,7 @@
 //! map). Every interrupt must be acknowledged with an end-of-interrupt write, or
 //! the LAPIC won't deliver the next one.
 
-const danos = @import("danos");
+const system = @import("system");
 const io = @import("io.zig");
 const paging = @import("paging.zig");
 
@@ -121,7 +121,7 @@ pub fn init() void {
 
     const msr = io.rdmsr(ia32_apic_base_msr);
     // Reach the LAPIC through the physmap (paging.init maps its page there).
-    base = @intCast(danos.physicalToVirtual(msr & 0xFFFFF000)); // physical base is bits 12+
+    base = @intCast(system.physicalToVirtual(msr & 0xFFFFF000)); // physical base is bits 12+
     io.wrmsr(ia32_apic_base_msr, msr | (1 << 11)); // global enable
 
     write(register_spurious, 0x100 | spurious_vector); // bit 8 = software enable