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/conformance.sh

@@ -38,6 +38,7 @@ SUITES=(
   "set_predicates:lib/prolog/tests/set_predicates.sx:pl-set-predicates-tests-run!"
   "char_predicates:lib/prolog/tests/char_predicates.sx:pl-char-predicates-tests-run!"
   "io_predicates:lib/prolog/tests/io_predicates.sx:pl-io-predicates-tests-run!"
+  "assert_rules:lib/prolog/tests/assert_rules.sx:pl-assert-rules-tests-run!"
 )
 
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