engine: boosted forms post text/sx, not urlencoded (SX-native write wire)

build-request-body's POST-form branch now serialises the form fields to a text/sx
body via the serialize primitive (content-type text/sx), instead of FormData ->
URLSearchParams -> urlencoded. A hydrated page posts SX; the host reads it via
host/sx-body / host/field (the server already accepts both — urlencoded stays the
no-engine / login-bootstrap fallback). Recompiled the web stack -> .sxbc.

Verified client-agnostically (no DOM, the user's preference): a new sxtp suite test
proves the wire contract serialize(engine) <-> host/sx-body(server) round-trips a
field dict losslessly, INCLUDING sx_content full of quotes/parens that would break a
naive encoder, plus host/field's content-type discrimination + urlencoded fallback
(sxtp 43/43). The DOM field-read (dom-query-all + .value) is the one irreducibly-
browser bit — left to a targeted Playwright smoke.

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

lib/host/tests/sxtp.sx

@@ -119,6 +119,29 @@
   (dream-status (sxtp/to-dream (sxtp/forbidden "no")))
   403)
 
+;; ── engine<->server write wire: serialize (engine) <-> host/sx-body (server) ──
+;; A boosted form posts (serialize {field->value}) as text/sx; the server reads it
+;; back with host/sx-body. This is the SX write wire, verified with NO DOM (client-
+;; agnostic): what the engine's serialize emits, host/sx-body must parse back
+;; losslessly — including sx_content full of the quotes/parens that would break a
+;; naive encoder. (The server side is what conformance can prove; the DOM field-read
+;; is the one irreducibly-browser bit, left to a Playwright smoke.)
+(define host-sx-wire-content "(article (h1 \"Title\") (p \"He said \\\"hi\\\" (x)\"))")
+(define host-sx-wire-req
+  (dream-request "POST" "/x" {:content-type "text/sx"}
+    (serialize {:title "Hi there" :sx_content host-sx-wire-content :status "published"})))
+(host-sx-test "sx-body round-trips a serialized field dict"
+  (get (host/sx-body host-sx-wire-req) "title") "Hi there")
+(host-sx-test "sx-body preserves quoted/parenthesised sx_content losslessly"
+  (get (host/sx-body host-sx-wire-req) "sx_content") host-sx-wire-content)
+(host-sx-test "field reads a text/sx body by content-type"
+  (host/field host-sx-wire-req "status") "published")
+(host-sx-test "field falls back to urlencoded form (the no-engine path)"
+  (host/field (dream-request "POST" "/x"
+                {:content-type "application/x-www-form-urlencoded"}
+                "title=From+Form&status=draft") "title")
+  "From Form")
+
 (define
   host-sx-tests-run!
   (fn