Placeholder discovery services and the -Ddiscovery build option

system/services/acpi and system/services/fdt exist as documented
placeholders (silent clean-exit mains; the headers say exactly what each
becomes and why). The build's -Ddiscovery=acpi|fdt option fills the
ramdisk's neutral 'discovery' slot — the device manager will spawn
"discovery" by that name in M20.3 and never learn which firmware it is
on (m19-m20-plan.md decision 7). x86 defaults to acpi; the aarch64
target flips the default when it lands.

system/services/fdt/fdt.zig

@@ -0,0 +1,34 @@
+//! /system/services/fdt — the devicetree discovery service: the ARM twin of the
+//! acpi service (docs/m19-m20-plan.md decision 7). **Placeholder: not
+//! implemented.** It exists so the build's `-Ddiscovery` option has both of its
+//! values from day one; the implementation lands with the Raspberry Pi
+//! bring-up (docs/arm.md).
+//!
+//! What it becomes: the per-firmware discoverer for boots that hand over a
+//! flattened device tree instead of ACPI tables. It claims the
+//! `devicetree-blob` node the kernel publishes (the FDT the loader received),
+//! walks the tree — pure data, no bytecode, so unlike the acpi service it
+//! needs no port grant and no interpreter — and, like any bus-shaped driver:
+//! `device_register`s what it finds (containment against the blob node's
+//! recorded apertures), reports each child to the device manager
+//! (`child_added`, identity = the node's `compatible` string), and stays
+//! resident under the manager's supervision (hello, restart, the usual
+//! contract).
+//!
+//! Known prerequisite recorded in the plan: `DeviceDescriptor`'s 8-byte `hid`
+//! cannot hold an FDT `compatible` string ("brcm,bcm2835-aux-uart") — identity
+//! widens before this file grows a body.
+
+const runtime = @import("runtime");
+
+pub fn main() void {
+    // Not implemented: exit cleanly and silently (a bare spawn by the
+    // initial-ramdisk sweep must not derange other tests' markers). The
+    // supervisor reads a clean exit as "meant to stop" — correct for a
+    // placeholder.
+}
+
+pub const panic = runtime.panic;
+comptime {
+    _ = &runtime.start._start; // pull the runtime entry shim into the image
+}