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.

test/chez/corpus.edn

@@ -3343,4 +3343,8 @@
   {:suite "host interop / Collection.contains" :label "value membership over vector/list/set" :expected "[true false true]" :actual "[(.contains [1 2 3] 2) (.contains (list :a :b) :z) (.contains #{1 2} 1)]"}
   {:suite "host interop / clojure.lang.Util" :label "Util/hash is Java hashCode" :expected "120" :actual "(clojure.lang.Util/hash \"x\")"}
   {:suite "deftype / IObj metadata" :label "deftype meta/withMeta govern (meta x)" :expected "{:a 1}" :actual "(do (deftype Sm [m] clojure.lang.IObj (meta [_] m) (withMeta [_ n] (Sm. n))) (meta (Sm. {:a 1})))"}
+  {:suite "collections / disj" :label "disj of nil is nil" :expected "nil" :actual "(disj nil :a)"}
+  {:suite "special forms / qualified" :label "clojure.core-qualified special form (from syntax-quote)" :expected "[:g 9]" :actual "(clojure.core/letfn [(g [x] [:g x])] (g 9))"}
+  {:suite "symbols / interning" :label "(str sym) is the symbol's interned name (identity-stable)" :expected "[true true]" :actual "(let [s (quote ?a)] [(identical? (name s) (str s)) (= (str s) \"?a\")])"}
+  {:suite "symbols / interning" :label "equal symbols share an interned name string" :expected "true" :actual "(let [a (quote ?foo) b (quote ?foo)] (identical? (name a) (name b)))"}
 ]