host: execute-fold — universality proven with a second fold (composition step 7)

The keystone validation of the universal-algebra thesis. lib/host/execute.sx is a SECOND
interpreter over the SAME seq/alt/each composition algebra as the render-fold — but a
different fold: leaves are EFFECTS, seq = steps in order, alt+when = branch, each =
for-each, and the accumulator is an effect log instead of an HTML string. It REUSES
compose.sx's shared machinery verbatim — host/comp--pred? (when), host/comp--field
(field/value), host/comp--source (each source) — so the predicate set, context-environment,
and iteration source are domain-agnostic; only the leaf semantics + accumulator are new.

KEYSTONE (tested): ONE (alt (when (has "auth") …) …) skeleton + ONE context folds two ways
— render picks the branch → "<b>in</b>", execute picks the SAME branch → {:verb "enter"}.
A publish workflow (validate → branch-on-status → notify-each) runs as one execute-fold over
a composition object. So the behaviour model (Slice 9) is "an execute-fold over a composition
object", not a separate system — the way the recursive tree proved recursion, this proves the
algebra is domain-agnostic. host/exec-run; 13/13 (new execute suite); wired into conformance
+ serve. Full host conformance 371/373 in 42s (warm); the 2 fails are the pre-existing
relate-picker pair.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/conformance.sh

@@ -93,6 +93,7 @@ MODULES=(
   "lib/host/feed.sx"
   "lib/host/relations.sx"
   "lib/host/compose.sx"
+  "lib/host/execute.sx"
   "lib/host/blog.sx"
   "lib/host/page.sx"
   "lib/host/server.sx"
@@ -108,6 +109,7 @@ SUITES=(
   "feed        host-fd-tests-run!  lib/host/tests/feed.sx"
   "relations   host-rl-tests-run!  lib/host/tests/relations.sx"
   "blog        host-bl-tests-run!  lib/host/tests/blog.sx"
+  "execute     host-ex-tests-run!  lib/host/tests/execute.sx"
   "session     host-se-tests-run!  lib/host/tests/session.sx"
   "page        host-pg-tests-run!  lib/host/tests/page.sx"
   "server      host-sv-tests-run!  lib/host/tests/server.sx"