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.

docs/m19-m20-plan.md

@@ -71,7 +71,11 @@ branch is green; keep branches; push everything.
    ("brcm,bcm2835-aux-uart") — identity widens before the fdt service exists;
    and cross-firmware surfaces are named by **domain, not firmware** (M21
    defines a *power* protocol, not an "ACPI events" protocol — PSCI/mailbox
-   sources feed the same subscribers on ARM).
+   sources feed the same subscribers on ARM). **Landed early (2026-07-13):**
+   both services exist as placeholders (system/services/acpi, system/services/
+   fdt) and the build's `-Ddiscovery=acpi|fdt` option fills the ramdisk's
+   neutral `discovery` slot — the manager will spawn "discovery" by that name
+   in M20.3 and never learn which firmware it is on.
 
 ## Status
 
@@ -94,7 +98,8 @@ branch is green; keep branches; push everything.
       are the assertion — xhci must come up spawned off a pci-bus report, and
       the suite must not be able to tell the difference. discovery.md updated.
 - [ ] **merge** `feat/pci-bus` → main, push.
-- [ ] **M20.1** — acpi service, parse only: kernel publishes `acpi-tables`
+- [ ] **M20.1** — acpi service, parse only (fills the existing placeholder at
+      system/services/acpi/acpi.zig): kernel publishes `acpi-tables`
       (decision 5) — memory over the table blobs, the broad io_port grant, and
       **the SCI as an irq resource** (from the FADT; unused until M21 but free
       to record now). The service claims it, maps the blobs, runs the shared