Make preemption knobs configurable; fix unused-variable warnings

Add `Config::alloc_interval()` and `Config::timeslice_cycles()` so
callers can tune preemption sensitivity at runtime. The values flow
through `RuntimeInner` and are written into per-scheduler-thread locals
via a new `configure_preempt()` call at thread startup, keeping the hot
path free of cross-thread coherency traffic.

Fix unused-variable warnings in channel.rs by inlining `current_pid()`
directly into `te!` macro arguments — since the no-op macro arm never
evaluates its argument, no binding is needed at the call site.

Clean up a handful of dead imports exposed by the refactor.

src/scheduler.rs

@@ -11,7 +11,7 @@ use crate::actor::current_pid;
 use crate::channel::Sender;
 use crate::pid::Pid;
 use crate::runtime::{
-    self, RuntimeInner, YieldIntent, ROOT_PID, RUNTIME,
+    self, RuntimeInner, YieldIntent, RUNTIME,
 };
 use crate::supervisor::Signal;
 use std::sync::Arc;