Real Thread/yield + Thread/interrupted (jolt-l2gc)

Thread/yield was a no-op and Thread/interrupted always returned false. Now:

- yield calls libc sched_yield (resolved once via the process symbols), so a
  spin loop relinquishes the CPU. Falls back to a zero-length park if the symbol
  can't be resolved.
- each OS thread carries an interrupt flag (a box, thread-local). currentThread
  returns a handle wrapping the calling thread's flag, so .interrupt from another
  thread sets the target's flag. .isInterrupted reads without clearing; the static
  Thread/interrupted reads and clears — JVM semantics.

Consolidates the Thread surface: currentThread + the instance methods live in
io.ss (where the handle and its classloader are built), the flag box + yield +
the interrupted static in host-static.ss. Unit cases cover yield, the read/clear
split, and a cross-thread interrupt over a future.

host/chez/run-unit.ss

@@ -56,6 +56,7 @@
   (hashtable-clear! ns-alias-table)
   (hashtable-clear! ns-refer-table)
   (hashtable-clear! ns-refer-all-table)
+  (clear-thread-interrupt!)   ; a case that set the runner thread's interrupt flag mustn't leak
   (when zj-ghier (jolt-invoke (var-deref "clojure.core" "reset!")
                    (var-cell-root zj-ghier) (jolt-invoke (var-deref "clojure.core" "make-hierarchy"))))
   (set-chez-ns! "user"))