Chez concurrency pt.1: real OS-thread futures + blocking promises (shared heap)

future/future-call run the body on a native thread (fork-thread) over the SAME
heap — JVM semantics, not Janet's isolated-heap snapshot. deref blocks on a
mutex+condition latch; timed (deref f ms val) uses an absolute deadline.
promise is a real blocking promise (deref parks until deliver), replacing the
Janet non-blocking atom shim. future?/future-done?/future-cancelled?/future-cancel
/realized? are native (the overlay versions read Janet map keys); re-asserted in
post-prelude over the overlay. pmap/pcalls/pvalues (overlay, over future) light
up for free.

Thread-safety this forces:
- atoms get a per-atom mutex; swap!/swap-vals! are a JVM-style CAS loop (f runs
  outside the lock, so a watch/validator can deref the same atom); reset!/
  compare-and-set! are atomic.
- the dynamic binding stack becomes a Chez thread-parameter, so each future/thread
  has its own; Chez inherits it at fork, giving binding conveyance (the shim also
  installs an explicit snapshot).
- Thread/sleep really sleeps now (a worker sleeping doesn't block the parent).

Re-minted the seed: future-call now resolves at compile time, so pmap compiles to
a var-deref instead of the host-static-call fallback that crashed. image.ss
unchanged.

Corpus: the 2 snapshot cases now match the JVM (shared) not Janet (isolated) —
allowlisted on both Chez gates; the two racy future-cancel cases allowlisted;
"promise undelivered" (blocks on JVM/Chez, profile :bucket :timeout) skipped like
:throws. Zero-Janet corpus 2544 -> 2569, 0 new divergences, floor raised. Full
Janet gate + JVM cert green.

jolt-byjr

test/chez/cli-test.janet

@@ -47,7 +47,28 @@
    ["(map eval [(quote (+ 1 1)) (quote (* 3 3))])" "(2 9)"]
    ["(defmacro add1 [x] (list (quote +) x 1)) (add1 10)" "11"]
    ["(defmacro twice [x] `(do ~x ~x)) (twice (+ 2 3))" "5"]
-   ["(defmacro m [x] `(+ ~x 1)) (m (m (m 0)))" "3"]])
+   ["(defmacro m [x] `(+ ~x 1)) (m (m (m 0)))" "3"]
+   # jolt-byjr: concurrency — futures/pmap/promise on real OS threads (shared heap).
+   ["(deref (future (+ 1 2)))" "3"]
+   ["@(future (* 6 7))" "42"]
+   ["(deref (future (mapv inc [1 2 3])))" "[2 3 4]"]
+   ["(let [f (future (+ 1 1))] [(deref f) (deref f)])" "[2 2]"]
+   ["(future? (future 1))" "true"]
+   ["(future? 42)" "false"]
+   ["(let [f (future 1)] (deref f) (future-done? f))" "true"]
+   ["(let [f (future 1)] (deref f) (realized? f))" "true"]
+   ["(let [f (future 42)] (deref f) (deref f 1000 :nope))" "42"]
+   ["(vec (pmap inc [1 2 3]))" "[2 3 4]"]
+   ["(vec (pmap + [1 2 3] [4 5 6]))" "[5 7 9]"]
+   ["(vec (pcalls (fn [] 1) (fn [] 2)))" "[1 2]"]
+   ["(vec (pvalues (+ 1 2) (+ 3 4)))" "[3 7]"]
+   # shared heap = JVM semantics (NOT Janet's isolated-heap snapshot): a captured
+   # atom is shared, so the future's swap! is visible to the parent.
+   ["(let [a (atom 0)] (deref (future (swap! a inc))) @a)" "1"]
+   ["(let [a (atom 0)] (dorun (pmap (fn [_] (swap! a inc)) [1 2 3 4])) @a)" "4"]
+   # promise blocks until delivered (JVM), unlike the Janet atom-shim.
+   ["(let [p (promise)] (deliver p 7) @p)" "7"]
+   ["(let [p (promise)] (future (deliver p :hi)) @p)" ":hi"]])
 
 (each [src want] cases
   (def [code out err] (joltc src))