Replace the strong-ref stub with genuine reclamation. The referent is held
through a weak-cons, so Chez's generational collector reclaims it once it is
otherwise unreachable (the pair's car becomes the bwp object, and .get returns
nil). A guardian registered on the referent makes the reference itself available
the instant its referent is collected, which ReferenceQueue.poll surfaces as
enqueued — the same hook clojure.core.cache's clear-soft-cache! drains.
Chez has no softer-than-weak reference, so a SoftReference clears on
unreachability rather than under memory pressure: a SoftCache evicts more eagerly
than the JVM's but is now real GC eviction, not an unbounded strong cache.
WeakReference gets the same (faithful) semantics. Added System/gc -> a full
collect so callers (and the queue) can force the cycle.
core.cache stays 1314/0/0 (its test values are immortal literals). Corpus row for
System/gc; make test + shakesmoke green.
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