Soft/WeakReference: real GC eviction via Chez weak pairs + guardians

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.

host/chez/host-static-methods.ss

@@ -106,6 +106,9 @@
   (list (cons "currentTimeMillis" (lambda () (->num (now-millis))))
         (cons "nanoTime" (lambda () (->num (* 1000000 (now-millis)))))
         (cons "exit" (lambda args (exit (if (null? args) 0 (jnum->exact (car args))))))
+        ;; System/gc -> a full Chez collection (so weak references clear and their
+        ;; guardians fire); Runtime.gc() routes here too.
+        (cons "gc" (lambda _ (collect (collect-maximum-generation)) jolt-nil))
         ;; wrapped in lambdas: the helpers are defined below, resolved at call time.
         (cons "getProperty" (lambda (k . d) (apply sys-get-property k d)))
         (cons "setProperty" (lambda (k v) (sys-set-property k v)))