otel P1: span model + API (with-span, parent stack, ring buffer)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/host/conformance.sh

@@ -99,6 +99,7 @@ MODULES=(
   "lib/host/page.sx"
   "lib/host/server.sx"
   "lib/host/ledger.sx"
+  "lib/host/otel.sx"
 )
 
 # Suites: NAME RUNNER-FN PATH
@@ -117,6 +118,7 @@ SUITES=(
   "page        host-pg-tests-run!  lib/host/tests/page.sx"
   "server      host-sv-tests-run!  lib/host/tests/server.sx"
   "ledger      host-lg-tests-run!  lib/host/tests/ledger.sx"
+  "otel        host-ot-tests-run!  lib/host/tests/otel.sx"
 )
 
 # Filter to a single suite if a name was given (filter the array itself so its