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/cpu.zig

@@ -4,7 +4,7 @@
 //! build.zig — no change to the generic code. Keep everything CPU-specific here
 //! (halt, the descriptor tables, later paging), and nothing generic.
 
-const danos = @import("danos");
+const system = @import("system");
 const parameters = @import("parameters");
 const gdt = @import("gdt.zig");
 const tss = @import("tss.zig");
@@ -132,7 +132,7 @@ pub fn init() void {
 /// Build the kernel's own page tables (with real permissions) and switch onto
 /// them. Needs the frame allocator and the boot info (for the memory map and the
 /// kernel's segment layout). Call once the frame allocator is up.
-pub fn enablePaging(allocFrame: *const fn () ?u64, freeFrame: *const fn (u64) void, boot_information: *const danos.BootInformation) void {
+pub fn enablePaging(allocFrame: *const fn () ?u64, freeFrame: *const fn (u64) void, boot_information: *const system.BootInformation) void {
     paging.init(allocFrame, freeFrame, boot_information);
 }