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/kernel.zig

@@ -1,5 +1,5 @@
 const std = @import("std");
-const danos = @import("danos");
+const system = @import("system");
 const parameters = @import("parameters");
 const architecture = @import("architecture");
 const console = @import("console.zig");
@@ -14,14 +14,14 @@ const initial_ramdisk = @import("initial-ramdisk");
 const platform = @import("platform");
 const tests = @import("tests.zig");
 const build_options = @import("build_options");
-const BootInformation = danos.BootInformation;
+const BootInformation = system.BootInformation;
 
 /// The calling convention used to enter the kernel. Pinned to SystemV explicitly:
 /// the bootloader is built for the UEFI target, whose C convention is Microsoft
 /// x64 (first argument in RCX), while the kernel is SystemV (first argument in
 /// RDI). Both sides reference this so the `boot_information` pointer lands in the
-/// register the other expects. `danos.kernel_abi` re-exports it to the loader.
-pub const kernel_abi = danos.kernel_abi;
+/// register the other expects. `system.kernel_abi` re-exports it to the loader.
+pub const kernel_abi = system.kernel_abi;
 
 // POST/checkpoint codes emitted to I/O port 0x80 at boot milestones — the
 // last-resort progress signal on a machine with no text output at all.
@@ -50,7 +50,7 @@ var ap_trampoline_page: u64 = 0;
 /// half. `boot_information` (also low) is reached through the physmap — its base is the
 /// same under the loader's bootstrap tables and the kernel's own.
 export fn kmainEntry(boot_information: *const BootInformation) callconv(kernel_abi) noreturn {
-    kmain(@ptrFromInt(danos.physicalToVirtual(@intFromPtr(boot_information))));
+    kmain(@ptrFromInt(system.physicalToVirtual(@intFromPtr(boot_information))));
 }
 
 fn kmain(boot_information: *const BootInformation) noreturn {
@@ -91,7 +91,7 @@ fn kmain(boot_information: *const BootInformation) noreturn {
 
     // Summarise the physical memory the loader handed us. The array is danos's
     // own MemoryRegion, so this is a plain slice — no firmware layout in sight.
-    const regions = @as([*]const danos.MemoryRegion, @ptrFromInt(danos.physicalToVirtual(boot_information.memory_map.regions)))[0..boot_information.memory_map.len];
+    const regions = @as([*]const system.MemoryRegion, @ptrFromInt(system.physicalToVirtual(boot_information.memory_map.regions)))[0..boot_information.memory_map.len];
     var usable_pages: u64 = 0;
     var reserved_pages: u64 = 0; // reserved RAM only — MMIO is device space, not RAM
     for (regions) |r| {
@@ -102,7 +102,7 @@ fn kmain(boot_information: *const BootInformation) noreturn {
         }
     }
     const total_pages = usable_pages + reserved_pages;
-    const total_bytes = total_pages * danos.page_size;
+    const total_bytes = total_pages * system.page_size;
     const gib = 1 << 30;
 
     log.write("\ndanos: physical memory\n");
@@ -277,7 +277,7 @@ fn kmain(boot_information: *const BootInformation) noreturn {
     // borrowing. This boot context then becomes the BSP's idle loop.
     if (boot_information.init_len != 0) {
         status("starting /sbin/init...\n");
-        const image = @as([*]const u8, @ptrFromInt(danos.physicalToVirtual(boot_information.init_base)))[0..boot_information.init_len];
+        const image = @as([*]const u8, @ptrFromInt(system.physicalToVirtual(boot_information.init_base)))[0..boot_information.init_len];
         process.spawnProcess(image, 4) catch |err| {
             statusPrint("/sbin/init failed to load: {s}\n", .{@errorName(err)});
         };
@@ -300,9 +300,9 @@ fn kmain(boot_information: *const BootInformation) noreturn {
 /// Spawn every program bundled in the initial_ramdisk as its own ring-3 process. A bad
 /// image or a program that fails to load is logged and skipped — the rest of the
 /// system still runs.
-fn startInitialRamdiskBinaries(boot_information: *const danos.BootInformation) void {
+fn startInitialRamdiskBinaries(boot_information: *const system.BootInformation) void {
     if (boot_information.initial_ramdisk_len == 0) return;
-    const image = @as([*]const u8, @ptrFromInt(danos.physicalToVirtual(boot_information.initial_ramdisk_base)))[0..boot_information.initial_ramdisk_len];
+    const image = @as([*]const u8, @ptrFromInt(system.physicalToVirtual(boot_information.initial_ramdisk_base)))[0..boot_information.initial_ramdisk_len];
     const rd = initial_ramdisk.Reader.init(image) orelse {
         status("initial_ramdisk: bad image, skipping\n");
         return;
@@ -386,11 +386,11 @@ fn statusPrint(comptime fmt: []const u8, args: anytype) void {
 
 /// Frames (4 KiB pages) to whole MiB.
 fn mib(pages: u64) u64 {
-    return pages * danos.page_size / (1024 * 1024);
+    return pages * system.page_size / (1024 * 1024);
 }
 
 fn kib(frames: u64) u64 {
-    return frames * danos.page_size / (1024);
+    return frames * system.page_size / (1024);
 }
 
 /// Report a CPU exception and halt **this core**. There's no fault recovery yet, so