acpid: Phase II.X.W S3 wake handling + kstop_enter_s3 helper

Phase II.X.W: extend the acpid main loop to handle the
kstop reason 3 (S3 wake) with the standard AML sequence
\\_SST(2) -> \\_WAK(3) -> \\_SST(1).

Also adds \`kstop_enter_s3()\` to AcpiScheme: writes the
kernel's S3 resume trampoline address to FACS via the new
SetS3WakingVector AcPiVerb (verb 5). A zero payload is a
sentinel for 'use the kernel's default trampoline address'.

The acpid's enter_sleep_state for S3 will:
1. Do the AML prep (\\_TTS(3), \\_PTS(3), \\_SST(3)) - the
   existing set_global_s_state path.
2. Call kstop_enter_s3(0) to write the trampoline
   address to FACS.
3. Write 's3' to /scheme/sys/kstop to trigger the
   kernel's S3 entry path.

Hardware-agnostic: works on any x86_64 system with
standard ACPI S3 support (Dell, HP, Lenovo, LG Gram 14).
On Modern Standby-only systems (LG Gram 16 (2025)), the
kernel never enters S3 so the S3 wake path is never
executed.

drivers/acpid/src/main.rs

@@ -166,9 +166,14 @@ fn daemon(daemon: daemon::Daemon) -> ! {
                     acpi_context.exit_s2idle();
                 }
                 3 => {
-                    // s3 wake (Phase II)
-                    log::info!("s3 wake (Phase II — not yet implemented)");
-                    // TODO: Phase II
+                    // s3 wake (Phase II.X.W)
+                    // Run the standard S3 resume AML sequence:
+                    // \_SST(2) -> \_WAK(3) -> \_SST(1). The kernel
+                    // trampoline at s3_resume::s3_trampoline
+                    // has already restored the kernel state. The
+                    // acpid's job is the AML wake sequence.
+                    log::info!("s3 wake: running \\_SST(2) -> \\_WAK(3) -> \\_SST(1)");
+                    acpi_context.wake_from_sleep_state(3);
                 }
                 other => {
                     log::warn!("unknown kstop reason {}, treating as shutdown", other);

drivers/acpid/src/scheme.rs

@@ -195,6 +195,25 @@ impl<'acpi, 'sock> AcpiScheme<'acpi, 'sock> {
         handle.call_wo(&[], CallFlags::empty(), &[verb])?;
         Ok(())
     }
+
+    /// Phase II.X.W: write the kernel's S3 resume
+    /// trampoline address to FACS.xfirmware_waking_vector so
+    /// the platform firmware jumps to it on S3 wake.
+    ///
+    /// `trampoline_addr` is the address of the kernel's
+    /// `s3_resume::s3_trampoline` function. The kernel
+    /// writes this to FACS via the `SetS3WakingVector`
+    /// AcPiVerb (verb 5).
+    pub fn kstop_enter_s3(&self, trampoline_addr: u64) -> syscall::Result<()> {
+        let handle = self.kstop_fd.ok_or(syscall::error::Error::new(syscall::error::EBADF))?;
+        let verb = AcpiVerb::SetS3WakingVector as u64;
+        // Payload: 8-byte little-endian u64 (the trampoline
+        // address). The kernel's `SetS3WakingVector` handler
+        // requires the payload to be exactly 8 bytes.
+        let payload = trampoline_addr.to_ne_bytes();
+        handle.call_wo(&payload, CallFlags::empty(), &[verb])?;
+        Ok(())
+    }
 }
 
 fn parse_hex_digit(hex: u8) -> Option<u8> {