mk: matche keyword pattern fix + classic puzzles

matche-pattern->expr now treats keyword patterns as literals that emit
themselves bare, rather than wrapping in (quote ...). SX keywords
self-evaluate to their string name; quoting them flips them to a
keyword type that does not unify with the bare-keyword usage at the
target site. This was visible only as a test failure on the diffo
clauses below — tightened the pattern rules.

tests/classics.sx exercises three end-to-end miniKanren programs:
  - 3-friend / 3-pet permutation puzzle
  - grandparent inference over a fact list (membero + fresh)
  - symbolic differentiation dispatched by matche on
    :x / (:+ a b) / (:* a b)

228/228 cumulative.

lib/minikanren/matche.sx

@@ -5,33 +5,30 @@
 ;;     (PATTERN2 g1 ...)
 ;;     ...)
 ;;
-;; Each clause unifies TARGET with PATTERN, introducing a fresh variable
-;; for every plain symbol in the pattern, and runs its goal body. The
-;; pattern grammar:
-;;
+;; Pattern grammar:
 ;;   _                 wildcard — fresh anonymous var
 ;;   x                 plain symbol — fresh var, bind by name
-;;   ATOM              literal (number, string, keyword, boolean) — must equal
+;;   ATOM              literal (number, string, boolean) — must equal
+;;   :keyword          keyword literal — emitted bare (keywords self-evaluate
+;;                     to their string name in SX, so quoting them changes
+;;                     their type from string to keyword)
 ;;   ()                empty list — must equal
 ;;   (p1 p2 ... pn)    list pattern — recurse on each element
 ;;
 ;; The macro expands to a `conde` whose clauses are
-;;   `((fresh (vars...) (== target pat-expr) body...))`.
+;;   `((fresh (vars-in-pat) (== target pat-expr) body...))`.
 ;;
-;; Fixed-length list patterns only — no rest patterns. To match "head + rest",
-;; use `(fresh (a d) (conso a d target) body)` directly.
+;; Repeated symbol names within a pattern produce the same fresh var, so
+;; they unify by `==`. Fixed-length list patterns only — head/tail
+;; destructuring uses `(fresh (a d) (conso a d target) body)` directly.
 ;;
 ;; Note: the macro builds the expansion via `cons` / `list` rather than a
-;; quasiquote — the quasiquote expander does not recurse into lambda
-;; bodies, which broke the natural `\`(matche-clause (quote ,target) cl)`
-;; spelling.
+;; quasiquote — quasiquote does not recurse into nested lambda bodies in
+;; SX, so `\`(matche-clause (quote ,target) cl)` left literal
+;; `(unquote target)` in the output.
 
 (define matche-symbol-var? (fn (s) (symbol? s)))
 
-(define
-  matche-collect-vars
-  (fn (pat) (matche-collect-vars-acc pat (list))))
-
 (define
   matche-collect-vars-acc
   (fn
@@ -43,6 +40,10 @@
         (reduce (fn (a p) (matche-collect-vars-acc p a)) acc pat))
       (:else acc))))
 
+(define
+  matche-collect-vars
+  (fn (pat) (matche-collect-vars-acc pat (list))))
+
 (define
   matche-pattern->expr
   (fn
@@ -51,6 +52,7 @@
       ((matche-symbol-var? pat) pat)
       ((and (list? pat) (empty? pat)) (list (quote list)))
       ((list? pat) (cons (quote list) (map matche-pattern->expr pat)))
+      ((keyword? pat) pat)
       (:else (list (quote quote) pat)))))
 
 (define