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.

system/kernel/architecture/x86_64/smp.zig

@@ -13,7 +13,7 @@
 //! Once a core has its own descriptor tables, LAPIC, and timer, it calls the generic
 //! scheduler entry and joins the run loop — mechanism here, policy there.
 
-const danos = @import("danos");
+const system = @import("system");
 const io = @import("io.zig");
 const gdt = @import("gdt.zig");
 const tss = @import("tss.zig");
@@ -85,7 +85,7 @@ fn arm() void {
     const start = @extern([*]const u8, .{ .name = "ap_trampoline_start" });
     const end = @extern([*]const u8, .{ .name = "ap_trampoline_end" });
     const len = @intFromPtr(end) - @intFromPtr(start);
-    const destination: [*]u8 = @ptrFromInt(danos.physicalToVirtual(tramp_physical));
+    const destination: [*]u8 = @ptrFromInt(system.physicalToVirtual(tramp_physical));
     @memcpy(destination[0..len], start[0..len]);
 }
 
@@ -95,7 +95,7 @@ fn arm() void {
 /// reported in — it's long past the trampoline by then, in the kernel image; a
 /// core that never answered is dead and can't be mid-climb.
 fn disarm() void {
-    const destination: [*]u8 = @ptrFromInt(danos.physicalToVirtual(tramp_physical));
+    const destination: [*]u8 = @ptrFromInt(system.physicalToVirtual(tramp_physical));
     @memset(destination[0..page_size], 0);
     paging.unmap(tramp_physical); // drop the transient low identity mapping
 }
@@ -107,7 +107,7 @@ fn disarm() void {
 fn param(comptime name: []const u8) *align(1) volatile u64 {
     const start = @intFromPtr(@extern([*]const u8, .{ .name = "ap_trampoline_start" }));
     const sym = @intFromPtr(@extern([*]const u8, .{ .name = name }));
-    return @ptrFromInt(danos.physicalToVirtual(tramp_physical + (sym - start)));
+    return @ptrFromInt(system.physicalToVirtual(tramp_physical + (sym - start)));
 }
 
 /// Wake the core with Local APIC id `apic_id` as dense CPU `index`, hand it