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/scoreboard.md

@@ -1,7 +1,7 @@
 # Prolog scoreboard
 
-**496 / 496 passing** (0 failure(s)).
-Generated 2026-04-25T13:49:43+00:00.
+**517 / 517 passing** (0 failure(s)).
+Generated 2026-04-25T14:12:52+00:00.
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -29,6 +29,7 @@ Generated 2026-04-25T13:49:43+00:00.
 | io_predicates | 24 | 24 | ok |
 | assert_rules | 15 | 15 | ok |
 | string_agg | 25 | 25 | ok |
+| advanced | 21 | 21 | ok |
 
 Run `bash lib/prolog/conformance.sh` to refresh. Override the binary
 with `SX_SERVER=path/to/sx_server.exe bash …`.