core.logic constraint layer: fixes for the CLP/unifier failures

Follow-on to the core.logic relational-engine work. These clear every crash in
core.logic's constraint-logic-programming and unifier layers (33 errors -> 0) and
most of the value mismatches; the suite goes 504 -> 523 passing assertions. All
are general gaps, not core.logic-specific.

- symbols intern their ns/name strings (JVM Symbol.intern .intern()s them): two
  separately-read `?a` symbols now share one name-string object. core.logic's
  non-unique lvars compare names by identity (via (str sym)), so without this a
  term's lvar and a constraint's lvar built from different `?a` reads never matched
  and constraints silently never fired.
- (str x) of a single arg returns its rendering directly instead of copying through
  string-append, and a symbol stringifies to its (interned) name — JVM (str x) is
  x.toString(). Needed for the identity comparison above.
- a clojure.core-qualified special form dispatches correctly: syntax-quote
  namespace-qualifies a macro like letfn to clojure.core/letfn (matching Clojure,
  where it's a macro), and the analyzer now maps that back to the special form
  instead of treating it as an invoke of a nil var. core.logic's fnc/defnc emit
  (clojure.core/letfn ...). Re-mint.
- (disj nil ...) is nil (JVM), instead of crashing in the set path — core.logic's
  constraint store does (disj (get km v) id) where the get can be nil.

corpus.edn: 4 JVM-certified rows. make test + shakesmoke green, 0 new divergences,
self-host fixpoint holds.

host/chez/transients.ss

@@ -171,8 +171,11 @@
 
 ;; persistent disj over sets (pset-disj already exists in collections.ss).
 (define (jolt-disj s . xs)
-  (meta-carry s
-    (let loop ((s s) (xs xs)) (if (null? xs) s (loop (pset-disj s (car xs)) (cdr xs))))))
+  ;; (disj nil ...) is nil on the JVM (disj is otherwise set-only).
+  (if (jolt-nil? s)
+      jolt-nil
+      (meta-carry s
+        (let loop ((s s) (xs xs)) (if (null? xs) s (loop (pset-disj s (car xs)) (cdr xs)))))))
 
 ;; --- see-through accessors ---------------------------------------------------
 (define (tvec-in-bounds? t i) (and (fixnum? i) (fx>=? i 0) (fx<? i (jolt-transient-n t))))