Implement clojure.core futures backed by Janet's ev/thread for genuine
parallelism (CPU-bound work can use a second core, unlike cooperative go
blocks):
- future / future-call, deref + (deref f timeout-ms timeout-val), future?,
future-done?, future-cancel, future-cancelled?; realized? on futures.
- A worker OS thread computes and marshals back a [:ok v]/[:error e] result
over a thread-chan; a parent-side collector fiber caches it and closes a
broadcast latch so any number of deref-ers unpark.
- Snapshot semantics: separate heaps mean the body + captured state are copied
to the worker and only the result is copied back (mutating a captured atom
does not propagate). Documented in README.
- future-cancel can't interrupt a Janet OS thread, so it marks the future
cancelled/done (deref throws, predicates flip) while the worker runs out.
clojure-test-suite baseline 3915 -> 3913: implementing future unskips
realized_qmark.cljc's (when-var-exists future ...) block, which depends on
JVM Thread/sleep + real thread interruption jolt can't provide; deref then
re-raises the unresolved-Thread/sleep error. Documented at the baseline.
Spec: test/spec/futures-spec.janet (18 cases).
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