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.

docs/framebuffer.md

@@ -12,7 +12,7 @@ exactly what `Console.pixel` does:
 self.rowPtr(y)[x] = color;   // system/kernel/console.zig
 ```
 
-Our `Framebuffer` struct (`system/danos.zig`) is the four facts you need to
+Our `Framebuffer` struct (`system/system.zig`) is the four facts you need to
 address it:
 
 | Field    | Meaning |