Split the system contract into boot-handoff / abi / device-abi

The `system` module (formerly `danos`) had become a grab-bag: it held the
loader<->kernel handoff *and* the kernel<->user ABI *and* the device wire types, in
one module three different audiences imported. Usage proved the seam — the
bootloader never touched the syscall/device ABI, and user space never touched the
boot handoff — so split it by audience, one module per contract:

  system/boot-handoff.zig       loader <-> kernel: BootInformation, Framebuffer,
                                MemoryMap, the VM layout + physicalToVirtual, kernel_abi
  system/abi.zig                kernel <-> user, core: SystemCall, mmap prot flags,
                                page_size, notify_badge_bit, ServiceId
  system/devices/device-abi.zig kernel <-> user, devices: DeviceDescriptor,
                                DeviceClass, ResourceDescriptor, ResourceKind, ...

device-abi is the devices sub-project's public interface, exposed as its own module
the way vfs exposes vfs-protocol — importable by user space, unlike the
kernel-internal device model it also feeds. That collapses a real duplication:
DeviceClass and ResourceKind were defined twice (device-model.zig and the contract,
kept "in sync by hand"); device-model now re-exports them from device-abi, so the
enum a driver matches on and the one the kernel classifies with are one type.

Each import now declares which contract it speaks: the bootloader imports only
boot-handoff; a driver only abi + device-abi (via the runtime); the kernel all
three. This also retires the `system` / `runtime.system` name overlap. page_size
lands in abi (it's part of the mmap contract user space aligns to); the bootloader
keeps its own local 4 KiB constant so it depends on nothing but the handoff.

All 21 importers rewired, docs updated to keep /system mapping to source. Build,
host tests, and the QEMU suite (36/36) all 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 system = @import("system");
+const boot_handoff = @import("boot-handoff");
 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(system.physicalToVirtual(tramp_physical));
+    const destination: [*]u8 = @ptrFromInt(boot_handoff.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(system.physicalToVirtual(tramp_physical));
+    const destination: [*]u8 = @ptrFromInt(boot_handoff.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(system.physicalToVirtual(tramp_physical + (sym - start)));
+    return @ptrFromInt(boot_handoff.physicalToVirtual(tramp_physical + (sym - start)));
 }
 
 /// Wake the core with Local APIC id `apic_id` as dense CPU `index`, hand it