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/acpi.md

@@ -16,7 +16,7 @@ the RSDT's address is a field *inside* the RSDP. The platform follows that point
 UEFI configuration table
    │  the loader reads the RSDP's physical address
    ▼
-BootInfo.acpi_rsdp   (u64, in the shared `danos` module)         system/danos.zig
+BootInfo.acpi_rsdp   (u64, in the shared `system` module)         system/system.zig
    │  the kernel forwards the whole BootInfo
    ▼
 platform.discover(boot_info, …)                                  system/devices/platform.zig
@@ -44,7 +44,7 @@ the [memory map](memory-map.md).
 
 The loader can't just call the device module: the bootloader binary and the kernel
 binary are compiled separately, and **the loader isn't linked against the `platform`
-module at all** (it imports only the shared `danos` module). So instead of a call, it
+module at all** (it imports only the shared `system` module). So instead of a call, it
 deposits a value in the handoff struct:
 
 ```zig