enumerate usable CPU cores from the MADT

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

src/kernel/main.zig

@@ -200,6 +200,10 @@ fn kmain(boot_info: *const BootInfo) noreturn {
             log.write("  console UART: none in SPCR -> legacy COM1\n");
         }
         log.print("  ioapic     : base 0x{x}, {d} inputs (masked); entry0 low 0x{x}\n", .{ ioapic_base, arch.ioapicEntryCount(), arch.ioapicEntryLow(0) });
+        const cores = platform.cpus();
+        log.print("  cpus       : {d} usable core(s); 1 running (BSP), {d} AP(s) parked (SMP bring-up pending)\n", .{ cores.len, if (cores.len > 0) cores.len - 1 else 0 });
+        if (platform.cpusDropped() > 0)
+            log.print("  cpus       : WARNING {d} core(s) beyond pool cap dropped\n", .{platform.cpusDropped()});
     } else |err| {
         log.print("\ndanos: device discovery failed: {s}\n", .{@errorName(err)});
     }