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/scoreboard.md

@@ -1,7 +1,7 @@
 # Prolog scoreboard
 
-**471 / 471 passing** (0 failure(s)).
-Generated 2026-04-25T13:21:37+00:00.
+**496 / 496 passing** (0 failure(s)).
+Generated 2026-04-25T13:49:43+00:00.
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -28,6 +28,7 @@ Generated 2026-04-25T13:21:37+00:00.
 | char_predicates | 27 | 27 | ok |
 | io_predicates | 24 | 24 | ok |
 | assert_rules | 15 | 15 | ok |
+| string_agg | 25 | 25 | ok |
 
 Run `bash lib/prolog/conformance.sh` to refresh. Override the binary
 with `SX_SERVER=path/to/sx_server.exe bash …`.