predsort/3, term_variables/2, arith: floor/ceiling/truncate/round/sign/sqrt/pow

- pl-eval-arith: add floor, ceiling, truncate, round, sqrt, sign, pow, integer,
  float, float_integer_part, float_fractional_part, **, ^ operators
- pl-collect-vars: helper that extracts unbound variables from a term (left-to-right,
  deduplicated by var id)
- term_variables/2: dispatches via pl-collect-vars, unifies second arg with var list
- pl-predsort-insert!: inserts one element into a sorted list using a 3-arg comparator
  predicate; deduplicates elements where comparator returns '='
- pl-predsort-build!: builds sorted list via fold over pl-predsort-insert!
- predsort/3: full ISO predsort — sorts and deduplicates a list using a caller-supplied
  predicate
- lib/prolog/tests/advanced.sx: 21 tests (12 arith, 5 term_variables, 4 predsort)
- conformance.sh: add advanced suite
- scoreboard: 517/517 (was 496/496)

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

lib/prolog/conformance.sh

@@ -40,6 +40,7 @@ SUITES=(
   "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!"
   "string_agg:lib/prolog/tests/string_agg.sx:pl-string-agg-tests-run!"
+  "advanced:lib/prolog/tests/advanced.sx:pl-advanced-tests-run!"
 )
 
 SCRIPT='(epoch 1)