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.

README.md

@@ -3,7 +3,7 @@ Codename: Shodan
 Version: 1
 
 A small operating system, written from scratch in Zig — a bootloader (`boot/`)
-and a microkernel (`system/kernel/`), sharing a neutral handoff contract (`system/danos.zig`).
+and a microkernel (`system/kernel/`), sharing a neutral handoff contract (`system/system.zig`).
 It boots x86-64 via UEFI, and so far has a framebuffer console, a physical frame
 allocator, its own paging with W^X permissions, interrupt/exception handling, a
 LAPIC timer, a kernel heap, a fixed-priority preemptive scheduler, and in-kernel IPC