host: Phase 2 complete — SXTP wire format + Dream bridge, 82/82

lib/host/sxtp.sx implements the host<->subsystem wire format per
applications/sxtp/spec.sx:

- message algebra (request/response/condition/event + status helpers
  ok/created/not-found/forbidden/invalid/fail) as string-keyed dicts;
  verb/status/type stored as symbols (ride the wire bare)
- codec: sxtp/serialize (dict -> text/sx list form, deterministic top-level
  field order, nested messages emitted in their own list form, no :msg leak)
  and sxtp/parse (text/sx -> dict via a deep keyword-token->string normaliser)
- Dream bridge: sxtp/from-dream (HTTP req -> SXTP req, method->verb,
  query->params) and sxtp/to-dream (SXTP resp -> HTTP resp, status->code,
  body serialised to text/sx)
- 39-test suite covering algebra, serialise/parse round-trip, mappings, bridge

Runtime notes: serialize renders string-keyed dicts as {:k v} and symbols
bare; parsed keyword tokens are a distinct type (not = to string literals) so
parse normalises; unquote-splicing is unreliable so the emitter is str-based.

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

lib/host/conformance.sh

@@ -56,6 +56,7 @@ MODULES=(
   "lib/dream/router.sx"
   "lib/host/handler.sx"
   "lib/host/middleware.sx"
+  "lib/host/sxtp.sx"
   "lib/host/router.sx"
   "lib/host/feed.sx"
 )
@@ -64,6 +65,7 @@ MODULES=(
 SUITES=(
   "handler     host-hd-tests-run!  lib/host/tests/handler.sx"
   "middleware  host-mw-tests-run!  lib/host/tests/middleware.sx"
+  "sxtp        host-sx-tests-run!  lib/host/tests/sxtp.sx"
   "router      host-rt-tests-run!  lib/host/tests/router.sx"
   "feed        host-fd-tests-run!  lib/host/tests/feed.sx"
 )