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.
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