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.

tests/runtime.rs

@@ -15,10 +15,7 @@
 //!   - Panic on one scheduler thread doesn't kill others
 
 use smarm::{channel, runtime::{Config, Runtime}, spawn, yield_now, JoinHandle};
-use std::sync::{
-    atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering},
-    Arc, Barrier,
-};
+use std::sync::{atomic::{AtomicBool, AtomicU64, Ordering}, Arc};
 use std::time::Duration;
 use std::collections::HashSet;