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.

jolt-core/jolt/analyzer.clj

@@ -611,6 +611,15 @@
       (quote-node form)
       (let [head (first items)
             hname (when (and (form-sym? head) (nil? (form-sym-ns head))) (form-sym-name head))
+            ;; a special-form head may arrive clojure.core-qualified: syntax-quote
+            ;; namespace-qualifies a macro like `letfn` to `clojure.core/letfn`
+            ;; (matching Clojure, where it is a macro), so a macro-emitted
+            ;; (clojure.core/letfn …) must still dispatch to the special form.
+            sf-name (or hname
+                        (when (and (form-sym? head)
+                                   (= "clojure.core" (form-sym-ns head))
+                                   (contains? handled (form-sym-name head)))
+                          (form-sym-name head)))
             shadowed (and hname (local? env hname))]
         (cond
           ;; Canonical order (Clojure/CLJS analyze-seq): macroexpand FIRST, then
@@ -619,7 +628,7 @@
           ;; the reference macroexpand1's isSpecial check — so a ns that redefs a
           ;; macro `def`/`and`/`or` (clojure.spec.alpha) keeps the special form `def`.
           (and (form-sym? head) (not shadowed)
-               (not (contains? handled hname)) (form-macro? ctx head))
+               (not (contains? handled sf-name)) (form-macro? ctx head))
             ;; defn/defn- expand to (def name (fn …)); carry the ORIGINAL form's
             ;; source offset onto the resulting def, since the macro builds a fresh
             ;; (def …) with no metadata. So the back end can register fn defs.
@@ -639,10 +648,10 @@
           ;; special-form heads are NOT shadowable (unlike macros): a local named
           ;; `if` does not change the meaning of (if …) in operator position, per
           ;; spec §3 and the reference. No (not shadowed) guard here.
-          (and hname (contains? handled hname))
+          (and sf-name (contains? handled sf-name))
             ;; stamp the form's source offset onto a top-level def so the back end
             ;; can register it (jv$ns$name -> source) for native stack traces.
-            (let [node (analyze-special ctx hname items env)
+            (let [node (analyze-special ctx sf-name items env)
                   p (form-position form)]
               (if (and p (= :def (:op node))) (assoc node :pos p) node))
           (and hname (not shadowed) (method-head? hname))