perf(context): pass actor sp through registers, not TLS

switch_to_actor takes the target sp in rdi and returns the actor's
next saved sp in rax (handed over by switch_to_scheduler's shim).
Deletes the ACTOR_SP thread-local and halves the helper calls per
one-way switch (2 -> 1); the scheduler loop also drops its
set_actor_sp/get_actor_sp TLS round-trips. SCHEDULER_SP stays: a
yielding actor at arbitrary call depth has no argument channel back.

asm before/after in outputs/history.md session 1-2. Tests: 354 pass.

src/runtime.rs

@@ -116,7 +116,7 @@ use crate::actor::{
     take_last_outcome, Actor, Outcome,
 };
 use crate::channel::Sender;
-use crate::context::{get_actor_sp, set_actor_sp, switch_to_actor};
+use crate::context::switch_to_actor;
 use crate::io::IoThread;
 use crate::monitor::{Down, DownReason, MonitorId};
 use crate::pid::Pid;
@@ -2228,7 +2228,6 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
             .current_pid_index
             .store(pid.index(), Ordering::Relaxed);
 
-        set_actor_sp(sp);
         set_current_pid(pid);
         crate::preempt::set_current_stop(stop_flag);
         crate::preempt::set_current_slot(slot as *const Slot);
@@ -2254,7 +2253,7 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         crate::causal::on_resume(slot);
 
         crate::te!(crate::trace::Event::Resume(pid));
-        unsafe { switch_to_actor() };
+        let saved_sp = unsafe { switch_to_actor(sp) };
 
         PREEMPTION_ENABLED.with(|c| c.set(false));
         // RFC 016 Chunk 2: charge the cycles this resume consumed to the actor
@@ -2268,7 +2267,6 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         crate::preempt::clear_current_slot();
 
         let intent = YIELD_INTENT.with(|c| c.get());
-        let saved_sp = get_actor_sp();
         slot.sp.store(saved_sp, Ordering::Relaxed);
         // RFC 019 §2: sampled high-water — one branch + at most one store
         // into the line the store above just dirtied. Relaxed and advisory;