IO predicates: term_to_atom/2, term_string/2, with_output_to/2, format/1,2, writeln/1

Adds 6 new built-in predicates to the Prolog runtime and 24 tests covering
term<->atom conversion (bidirectional), output capture, format directives (~w/~a/~d/~n/~~).
456/456 tests passing.

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

lib/prolog/conformance.sh

@@ -37,6 +37,7 @@ SUITES=(
   "meta_call:lib/prolog/tests/meta_call.sx:pl-meta-call-tests-run!"
   "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!"
 )
 
 SCRIPT='(epoch 1)