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/drivers.md

@@ -22,7 +22,7 @@ say.*
 
 ## The capability: claim before touch
 
-The five driver syscalls (`system/danos.zig`, dispatched in `system/kernel/process.zig`):
+The five driver syscalls (`system/system.zig`, dispatched in `system/kernel/process.zig`):
 
 | # | Call | Meaning |
 |---|------|---------|