fix(tls): actor-side thread-local accessors are #[inline(never)] + fence — LLVM caches TLS addresses across context switches (finding 19)

Without LTO (any downstream `cargo build --release`), LLVM keeps `%fs:0` in a
callee-saved register across `switch_to_scheduler`; after the actor migrates,
`ACTOR_DONE`/`PREEMPTION_ENABLED`/`CURRENT_PID` etc. hit the OLD thread's TLS.
8 multi-thread tests aborted with "scheduler resumed a done actor" (error 5).
Thin LTO only worked because it picked the local-exec model.

- context.rs: module doc "Thread-locals and migration" (the rule), tls_fence()
- every TLS accessor reachable from an actor stack is out of line:
  actor::{current_pid, publish_outcome (new)}, preempt::{maybe_preempt,
  check_cancelled, current_slot_ptr, note_*, preemption_swap/enabled (new;
  replaces direct PREEMPTION_ENABLED.with in NoPreempt/RawMutex/with_runtime/
  trace/debug asserts)}, context::{get,set}_scheduler_sp, runtime::{sched_slot,
  slot_push, set_yield_intent}, causal, trace
- raw_mutex order checks: out of line only under debug_assertions
- Cargo.toml: [profile.reltest] = release without LTO; 41 test bins link in
  ~1.5 s each instead of ~12 s (suite 11 min -> 1.5 min) AND it is the
  regression oracle for this bug: `cargo test --profile reltest`
Both profiles: 41/41 + doctests green.

src/causal.rs

@@ -274,7 +274,9 @@ mod inner {
 
     /// The experiment-active path, kept out of the inlined fast path.
     #[cold]
+    #[inline(never)]
     fn cold_check(exp: u64) {
+        crate::context::tls_fence();
         let slot = preempt::current_slot_ptr();
         if slot.is_null() {
             return;
@@ -367,8 +369,9 @@ mod inner {
     /// - Entering the target site: re-arm the sample clock, so time spent
     ///   *before* the site can never be attributed to it by the first
     ///   in-site check (the symmetric over-attribution).
-    #[inline]
+    #[inline(never)]
     fn site_transition(slot: *const crate::runtime::Slot, old: u32, new: u32) {
+        crate::context::tls_fence();
         let exp = EXPERIMENT.load(Ordering::Relaxed);
         if exp == 0 || old == new {
             return;