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.

library/runtime/ipc.zig

@@ -3,7 +3,7 @@
 //! reached this way. The server side (`replyWait`, which returns two values) is
 //! added with the first server binary.
 
-const system = @import("system");
+const abi = @import("abi");
 const sc = @import("system-call.zig");
 
 /// A small-int handle into the calling process's handle table.
@@ -30,13 +30,13 @@ pub fn createEndpoint() ?Handle {
 }
 
 /// Publish endpoint `h` under a well-known service id so other processes find it.
-pub fn register(id: system.ServiceId, h: Handle) bool {
+pub fn register(id: abi.ServiceId, h: Handle) bool {
     return !failed(sc.systemCall2(.ipc_register, @intFromEnum(id), h));
 }
 
 /// Find the endpoint published under `id`, installing a handle to it in this
 /// process.
-pub fn lookup(id: system.ServiceId) ?Handle {
+pub fn lookup(id: abi.ServiceId) ?Handle {
     const r = sc.systemCall1(.ipc_lookup, @intFromEnum(id));
     return if (failed(r)) null else r;
 }
@@ -53,7 +53,7 @@ pub fn call(h: Handle, message: []const u8, reply: []u8) CallError!usize {
 /// Set in `Received.badge` when what arrived is an asynchronous notification — a
 /// bound device interrupt — rather than a client's message. The low bits carry the
 /// GSI. See `isNotification`.
-pub const notify_badge_bit: u64 = system.notify_badge_bit;
+pub const notify_badge_bit: u64 = abi.notify_badge_bit;
 
 /// The result of a `replyWait`: the request length and the sender's badge (a
 /// task id, or an IRQ notification if the high bit is set).
@@ -84,7 +84,7 @@ pub fn replyWait(h: Handle, reply: []const u8, receive: []u8) Received {
     asm volatile ("syscall"
         : [rax] "={rax}" (rax),
           [rdx] "+{rdx}" (rdx),
-        : [n] "{rax}" (@intFromEnum(system.SystemCall.ipc_reply_wait)),
+        : [n] "{rax}" (@intFromEnum(abi.SystemCall.ipc_reply_wait)),
           [a0] "{rdi}" (h),
           [a1] "{rsi}" (@intFromPtr(reply.ptr)),
           [a3] "{r10}" (@intFromPtr(receive.ptr)),