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/run-corpus-prelude.janet

@@ -91,7 +91,22 @@
    "atom?" true
    # Same atom-class gap, via the folded-in conformance case (jolt-ohtd):
    # (instance? clojure.lang.Atom (atom 1)).
-   "instance? Atom" true})
+   "instance? Atom" true
+   # concurrency (jolt-byjr): the Chez runtime now uses real OS-thread futures
+   # sharing the heap = JVM semantics, NOT Janet's isolated-heap snapshot. So a
+   # captured atom is shared (the corpus :expected is the Janet snapshot value).
+   # Deliberate per jvm-parity-north-star. Same allowlist as the zero-Janet gate.
+   "captured atom is snapshotted, not shared" true   # Chez 1 (shared) vs Janet 0
+   "snapshot semantics" true                          # pmap: Chez 2 (shared) vs Janet 0
+   # future-cancel of a trivial body races the worker under real threads (cancel
+   # usually loses -> false), like the JVM; the Janet :expected relies on its
+   # cooperative scheduler. Flaky/divergent.
+   "cancel an in-flight future returns true" true
+   "future-cancelled? after cancel" true})
+
+# Cases that BLOCK forever on a shared-heap / JVM host (profile.edn :bucket
+# :timeout) — skip, like :throws, so a hung per-case process can't stall the gate.
+(def skip-blocking {"promise undelivered" true})
 
 (def ctx (d/make-ctx))
 
@@ -130,7 +145,7 @@
 (def t1 (os/clock))
 (each row cases
   (def {:expected e :actual a :label l} row)
-  (if (= e :throws)
+  (if (or (= e :throws) (get skip-blocking l))
     nil  # :throws error-semantics aren't modeled here; skip (counted out of run)
     (let [src (string "(= " e " " a ")")
           res (d/eval-e-with-prelude ctx src prelude-path)]