feat: add kernel ACPI support for querying battery percentage, add battery indicator to desktop, move log state to 0:/state directory

programs/include/montauk/syscall.h

@@ -639,6 +639,13 @@ namespace montauk {
         return (int)syscall1(montauk::abi::SYS_BTINFO, (uint64_t)buf);
     }
 
+    // ACPI battery status. Returns 0 on success, or -1 on a machine with no
+    // battery device at all -- which is the signal not to draw an indicator.
+    // A laptop with the pack removed still returns 0, with present == 0.
+    inline int battery_info(montauk::abi::BatteryInfo* out) {
+        return (int)syscall1(montauk::abi::SYS_BATTERYINFO, (uint64_t)out);
+    }
+
     // Wi-Fi. scan() runs a full channel sweep and blocks until it finishes or
     // timeoutMs elapses, then fills buf with the networks seen; it returns the
     // number of entries written, or -1 when no adapter is ready.