enumerate usable CPU cores from the MADT

Keep each Local APIC's id and expose platform.cpus().

docs/smp.md

@@ -114,7 +114,10 @@ active reconsideration in favour of resilience — see [vision.md](vision.md).)
 Whatever the top goal, the *sequence* is the same and seL4 validates starting simple:
 
 1. **Enumerate cores** — needs [device discovery](discovery.md) (ACPI MADT on x86,
-   device tree on ARM). SMP is a concrete consumer of that work.
+   device tree on ARM). SMP is a concrete consumer of that work. **Done on x86:** the
+   MADT parse records every usable Local APIC — with the `apic_id` an AP wake targets —
+   and `platform.cpus()` returns the list (see [discovery.md](discovery.md)). The boot
+   log reports the count; the ARM (device-tree) path still needs it.
 2. **Wake the APs** — INIT–SIPI–SIPI on x86; PSCI/spin-tables on ARM. Each core brings
    up its own tables, timer, and idle task.
 3. **Start with a big kernel lock.** It's a legitimate first design, not a shortcut —