gen_server: TimerHandle arm_after/cancel + Sys::Timer dispatch (RFC 015 §4.3, §5)

src/timer.rs

@@ -75,6 +75,17 @@ pub enum Reason {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct TimerId(u64);
 
+impl TimerId {
+    /// Wrap a raw value. Crate-internal: the gen_server timer layer mints its
+    /// own loop-local `TimerId`s (the public ids it hands out, decoupled from
+    /// the per-re-arm substrate `seq`) and maps them to live substrate ids.
+    /// These local ids are only ever resolved through that layer's registry —
+    /// never passed back to [`Timers::cancel`] — so the two id roles do not mix.
+    pub(crate) fn from_raw(v: u64) -> Self {
+        TimerId(v)
+    }
+}
+
 /// Callback the scheduler invokes when a `WaitTimeout` entry pops.
 ///
 /// Implementors: do not touch `SchedulerState` other than via the public