redox_syscall: add EnterS2Idle/ExitS2Idle AcpiVerb variants (Phase I/J)

Phase I/J: hardware-agnostic s2idle / Modern Standby
support. These AcpiVerb variants are needed on any
platform with Modern Standby firmware (Dell, HP, Lenovo,
LG Gram, etc.), not just LG Gram. They mirror Linux 7.1:

* EnterS2Idle (= 3) — s2idle_enter() in
  kernel/power/suspend.c:91
* ExitS2Idle (= 4) — s2idle_wake() in
  kernel/power/suspend.c:133

The version field stays at upstream 0.8.1. We do NOT
bump the version — this is the durable overlay pattern
(per AGENTS.md 'GOLDEN RULE — Red Bear adapts to
upstream, never the reverse'). Periodic rebase via
'git fetch upstream && git rebase upstream/master' is
the workflow when upstream changes.

The patch file backing this commit is at
local/patches/syscall/P1-acpiverb-enter-exit-s2idle.patch
in the outer RedBear-OS repo.

src/flag.rs

@@ -310,12 +310,29 @@ pub enum AcpiVerb {
     ReadRxsdt = 1,
     // no payload, just returns 0 or 1
     CheckShutdown = 2,
+    /// Red Bear OS extension (Phase I): acpid requests the kernel
+    /// enter s2idle (Modern Standby / S0ix). The kernel sets
+    /// `S2IDLE_REQUESTED`; the idle path calls `mwait_loop()`. Read
+    /// payload (1 byte) returns the *previous* value of the flag.
+    /// Write payload is opaque (ignored by current kernel).
+    /// Mirrors Linux 7.1 `s2idle_enter()` in
+    /// `kernel/power/suspend.c:91`. Hardware-agnostic — works on
+    /// any platform with Modern Standby firmware (Dell, HP, Lenovo,
+    /// LG Gram, etc.), not just LG Gram.
+    EnterS2Idle = 3,
+    /// Red Bear OS extension (Phase I): acpid signals s2idle
+    /// exit. Kernel clears `S2IDLE_REQUESTED`. Read payload (1
+    /// byte) always returns 0. Mirrors Linux 7.1 `s2idle_wake()` in
+    /// `kernel/power/suspend.c:133`. Hardware-agnostic.
+    ExitS2Idle = 4,
 }
 impl AcpiVerb {
     pub const fn try_from_raw(value: u64) -> Option<Self> {
         Some(match value {
             1 => Self::ReadRxsdt,
             2 => Self::CheckShutdown,
+            3 => Self::EnterS2Idle,
+            4 => Self::ExitS2Idle,
             _ => return None,
         })
     }