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/acpi/acpi.zig

@@ -0,0 +1,30 @@
+//! /system/services/acpi — the ACPI discovery service: the x86 firmware
+//! interpreter, moved out of ring 0 (docs/m19-m20-plan.md, M20). **Placeholder:
+//! not implemented until M20.1** — it exists so the build's `-Ddiscovery`
+//! option has both of its values and the ramdisk's neutral `discovery` slot is
+//! wired before the implementation lands.
+//!
+//! What it becomes (the plan's decisions 5 and 7): claim the `acpi-tables`
+//! node the kernel publishes (table blobs + the broad io_port grant + the SCI),
+//! map the tables, and run the **shared AML module** in ring 3 behind a `Hal`
+//! backed by `mmio_map` and `io_read`/`io_write` — the interpreter cannot tell
+//! it moved. Then the bus-driver shape: `device_register` the namespace
+//! devices (`_HID`, `_CRS` resources, containment against the node's
+//! apertures), report each to the device manager, stay resident under its
+//! supervision. M21 grows the event side on the same claim: the SCI, PM1 fixed
+//! events, GPEs, Notify — published through the domain-named power protocol,
+//! never an "ACPI events" protocol.
+
+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
+}