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/values.ss

@@ -47,9 +47,21 @@
 (define (keyword? x) (keyword-t? x))
 
 ;; --- symbols: ns + name + meta; NOT interned (meta varies), = by ns/name ------
+;; The ns/name STRINGS are pooled (like JVM Symbol.intern, which .intern()s them):
+;; two separately-read `?a` symbols share one name-string object, so code that
+;; compares symbol names by identity (core.logic's non-unique lvar equality, via
+;; (str sym)) behaves like the JVM.
+(define symbol-string-pool (make-hashtable string-hash string=?))
+(define (intern-symbol-string s)
+  (if (string? s)
+      (or (hashtable-ref symbol-string-pool s #f)
+          (begin (hashtable-set! symbol-string-pool s s) s))
+      s))
 (define-record-type symbol-t (fields ns name meta) (nongenerative symbol-v1))
-(define (jolt-symbol ns name) (make-symbol-t ns name jolt-nil))
-(define (jolt-symbol/meta ns name meta) (make-symbol-t ns name meta))
+(define (jolt-symbol ns name)
+  (make-symbol-t (intern-symbol-string ns) (intern-symbol-string name) jolt-nil))
+(define (jolt-symbol/meta ns name meta)
+  (make-symbol-t (intern-symbol-string ns) (intern-symbol-string name) meta))
 (define (jolt-symbol? x) (symbol-t? x))
 
 ;; chars/strings: Chez natives (strings treated immutable).