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>

shared/static/wasm/sx/boot-helpers.sx

@@ -269,16 +269,28 @@
                   (let
                     ((fd (host-new "FormData" el)))
                     (dict "url" url "body" fd "content-type" nil))
+                  ;; SX-native wire: serialise the form fields to a text/sx body
+                  ;; (the host reads it via host/sx-body / host/field). A hydrated
+                  ;; page posts SX, not urlencoded; the server still accepts
+                  ;; urlencoded for the no-engine fallback. See plans/
+                  ;; relations-as-posts.md ("SX all the way out").
                   (let
-                    ((fd (host-new "FormData" el))
-                      (params (host-new "URLSearchParams" fd)))
+                    ((payload
+                       (reduce
+                         (fn (acc f)
+                           (let ((nm (dom-get-attr f "name")))
+                             (if (and nm (not (= nm "")))
+                               (assoc acc nm (or (host-get f "value") ""))
+                               acc)))
+                         (dict)
+                         (dom-query-all el "input, textarea, select"))))
                     (dict
                       "url"
                       url
                       "body"
-                      (host-call params "toString")
+                      (serialize payload)
                       "content-type"
-                      "application/x-www-form-urlencoded"))))
+                      "text/sx; charset=utf-8"))))
               (dict "url" url "body" nil "content-type" nil))))))
     (define abort-previous-target (fn (el) nil))
     (define abort-previous (fn (el) nil))