sub_atom/5 (non-det substring) + aggregate_all/3 (count/bag/sum/max/min/set)

Adds two new builtins to lib/prolog/runtime.sx:

- sub_atom/5: non-deterministic substring enumeration. Iterates all
  (start, length) pairs over the atom string, tries to unify Before,
  Length, After, SubAtom for each candidate. Uses CPS loop helpers
  pl-substring, pl-sub-atom-try-one!, pl-sub-atom-loop!. Fixed trail
  undo semantics: only undo on backtrack (k returns false), not on success.

- aggregate_all/3: collects all solutions via pl-collect-solutions then
  reduces. Templates: count, bag(T), sum(E), max(E), min(E), set(T).
  max/min fail on empty; count/bag/sum/set always succeed.

New test suite lib/prolog/tests/string_agg.sx: 25 tests, all passing.
Total conformance: 496/496.

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

lib/prolog/conformance.sh

@@ -39,6 +39,7 @@ SUITES=(
   "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!"
+  "string_agg:lib/prolog/tests/string_agg.sx:pl-string-agg-tests-run!"
 )
 
 SCRIPT='(epoch 1)