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/memory-map.md

@@ -27,7 +27,7 @@ danos's own neutral format, and the kernel only ever sees that.**
 
 ## The neutral format
 
-Defined in `system/danos.zig`, the shared loader↔kernel contract:
+Defined in `system/system.zig`, the shared loader↔kernel contract:
 
 ```zig
 pub const MemoryKind = enum(u32) {
@@ -121,7 +121,7 @@ The kernel receives a plain array and reads it with zero UEFI knowledge:
 
 ```zig
 const mm = boot_info.memory_map;
-const regions = @as([*]const danos.MemoryRegion, @ptrFromInt(mm.regions))[0..mm.len];
+const regions = @as([*]const system.MemoryRegion, @ptrFromInt(mm.regions))[0..mm.len];
 for (regions) |r| {
     if (r.kind == .usable) usable_pages += r.pages;
 }