Add :- to op table (prec 1200 xfx); enable assert/asserta/assertz with rule terms

- parser.sx: add (":-" 1200 "xfx") to pl-op-table so (head :- body) parses
  inside paren expressions (parens reset prec to 1200, allowing xfx match)
- parser.sx: extend pl-token-op to accept "op" token type, not just "atom",
  since the tokenizer emits :- as {:type "op" :value ":-"}
- tests/assert_rules.sx: 15 new tests covering assertz/asserta with rule
  terms, conjunction in rule body, recursive rules, and ordering
- conformance.sh: wire in assert_rules suite
- 456 → 471 tests, all passing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/prolog/parser.sx

@@ -101,6 +101,7 @@
     (list "-" 500 "yfx")
     (list "*" 400 "yfx")
     (list "/" 400 "yfx")
+    (list ":-" 1200 "xfx")
     (list "mod" 400 "yfx")))
 
 (define
@@ -126,7 +127,7 @@
           (let
             ((info (pl-op-lookup ",")))
             (if (nil? info) nil (cons "," info))))
-        ((= ty "atom")
+        ((or (= ty "atom") (= ty "op"))
           (let
             ((info (pl-op-lookup vv)))
             (if (nil? info) nil (cons vv info))))