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/console.zig

@@ -14,7 +14,7 @@
 //! never assumes a display exists.
 
 const std = @import("std");
-const danos = @import("danos");
+const system = @import("system");
 
 /// The one framebuffer console, valid only when `con_present`.
 var con: Console = undefined;
@@ -22,7 +22,7 @@ var con_present: bool = false;
 
 /// Set up the console over `fb`, or mark it absent if there's no usable
 /// framebuffer. Clears the screen when present.
-pub fn init(fb: danos.Framebuffer) void {
+pub fn init(fb: system.Framebuffer) void {
     if (!fb.present()) {
         con_present = false;
         return;
@@ -58,7 +58,7 @@ const glyph_bytes = glyph_h; // 8 pixels wide => 1 byte per row
 const glyph_data = 32; // PSF2 header size
 
 pub const Console = struct {
-    fb: danos.Framebuffer,
+    fb: system.Framebuffer,
     cols: u32,
     rows: u32,
     col: u32 = 0,
@@ -66,12 +66,12 @@ pub const Console = struct {
     fg: u32 = 0x00c8_c8c8, // light grey
     bg: u32 = 0x0000_0000, // black
 
-    pub fn init(fb: danos.Framebuffer) Console {
+    pub fn init(fb: system.Framebuffer) Console {
         // Reach the framebuffer through the physmap, so the pointer stays valid
         // once the low identity map is gone. The base is mapped by both the
         // loader's bootstrap tables and paging.init.
         var mapped = fb;
-        if (fb.base != 0) mapped.base = danos.physicalToVirtual(fb.base);
+        if (fb.base != 0) mapped.base = system.physicalToVirtual(fb.base);
         return .{
             .fb = mapped,
             .cols = fb.width / glyph_w,