fed-sx-m1: 8b-bridge cleanup — remove dead helpers + duplicate test

Step 8b-bridge was actually completed in 0f85bd96 (Step 8b-start) using
er-request-dict-to-proplist / er-proplist-to-dict plus er-spawn-fun to
host the handler inside a real Erlang process. My previous commit
(31ff1e6a) shipped a parallel set of helpers (er-http-req-of-sx,
er-http-resp-to-sx and friends) plus a duplicate test under
next/tests/http_listen_bridge.sh — the BIF body never referenced them,
so they sat in runtime.sx as dead code while http_marshal.sh already
covered the live marshalers.

This commit:
  - deletes the 8 dead helpers from lib/erlang/runtime.sx
  - deletes the duplicate next/tests/http_listen_bridge.sh
  - rewrites next/README.md substrate gap #3 to name the helpers and
    tests that are actually live

No behaviour change. Erlang conformance still 761/761; http_listen_bif
5/5, http_route 11/11, http_publish_fold 10/10, http_marshal 10/10.

lib/erlang/runtime.sx

@@ -1578,147 +1578,6 @@
 ;; entry is keyed by "Module/Name/Arity"; multi-arity BIFs register
 ;; once per arity. Called eagerly at the end of runtime.sx so the
 ;; registry is ready before any erlang-eval-ast call.
-(define
-  er-binary->string
-  (fn (b) (list->string (map integer->char (get b :bytes)))))
-
-;; Register everything at load time.
-(define
-  string->er-binary
-  (fn (s) (er-mk-binary (map char->integer (string->list s)))))
-
-(define
-  er-mk-proplist
-  (fn
-    (pairs)
-    (let
-      ((out (er-mk-nil)))
-      (for-each
-        (fn
-          (i)
-          (let
-            ((idx (- (- (len pairs) 1) i)))
-            (let
-              ((pair (nth pairs idx)))
-              (set!
-                out
-                (er-mk-cons
-                  (er-mk-tuple
-                    (list
-                      (er-mk-atom (nth pair 0))
-                      (nth pair 1)))
-                  out)))))
-        (range 0 (len pairs)))
-      out)))
-
-(define
-  er-proplist-get
-  (fn
-    (plist key default)
-    (cond
-      (er-nil? plist)
-      default
-      (er-cons? plist)
-      (let
-        ((head (get plist :head)) (tail (get plist :tail)))
-        (let
-          ((match? (cond (not (er-tuple? head)) false :else (let ((es (get head :elements))) (cond (< (len es) 2) false (not (er-atom? (nth es 0))) false :else (= (get (nth es 0) :name) key))))))
-          (cond
-            match?
-            (nth (get head :elements) 1)
-            :else (er-proplist-get tail key default))))
-      :else default)))
-
-(define
-  er-http-headers-of-sx
-  (fn
-    (hdrs)
-    (cond
-      (not (= (type-of hdrs) "dict"))
-      (er-mk-nil)
-      :else (let
-        ((ks (keys hdrs)) (out (er-mk-nil)))
-        (for-each
-          (fn
-            (i)
-            (let
-              ((idx (- (- (len ks) 1) i)))
-              (let
-                ((k (nth ks idx)))
-                (let
-                  ((v (get hdrs k)))
-                  (set!
-                    out
-                    (er-mk-cons
-                      (er-mk-tuple
-                        (list
-                          (string->er-binary k)
-                          (string->er-binary
-                            (if (= (type-of v) "string") v ""))))
-                      out))))))
-          (range 0 (len ks)))
-        out))))
-
-(define
-  er-http-headers-to-sx
-  (fn
-    (hdrs)
-    (let
-      ((pairs (er-cons-to-sx-list hdrs)) (out {}))
-      (for-each
-        (fn
-          (i)
-          (let
-            ((p (nth pairs i)))
-            (cond
-              (not (= (type-of p) "list"))
-              nil
-              (< (len p) 2)
-              nil
-              :else (dict-set! out (nth p 0) (nth p 1)))))
-        (range 0 (len pairs)))
-      out)))
-
-(define
-  er-http-req-of-sx
-  (fn
-    (req-dict)
-    (let
-      ((s (fn (v) (if (= (type-of v) "string") v ""))))
-      (let
-        ((method (s (get req-dict "method")))
-          (path (s (get req-dict "path")))
-          (query (s (get req-dict "query")))
-          (body (s (get req-dict "body")))
-          (hdrs-d (get req-dict "headers")))
-        (er-mk-proplist
-          (list
-            (list "method" (string->er-binary method))
-            (list "path" (string->er-binary path))
-            (list "query" (string->er-binary query))
-            (list "headers" (er-http-headers-of-sx hdrs-d))
-            (list "body" (string->er-binary body))))))))
-
-(define
-  er-http-resp-to-sx
-  (fn
-    (resp)
-    (let
-      ((status-v (er-proplist-get resp "status" 200))
-        (headers-v (er-proplist-get resp "headers" (er-mk-nil)))
-        (body-v (er-proplist-get resp "body" (string->er-binary ""))))
-      (let
-        ((status (cond (= (type-of status-v) "number") status-v :else 200))
-          (body
-            (cond
-              (er-binary? body-v)
-              (er-binary->string body-v)
-              (= (type-of body-v) "string")
-              body-v
-              :else ""))
-          (hdrs (er-http-headers-to-sx headers-v)))
-        {:body body :headers hdrs :status status}))))
-
 (define
   er-bif-http-listen
   (fn
@@ -1731,14 +1590,10 @@
         (not (er-fun? handler))
         (raise (er-mk-error-marker (er-mk-atom "badarg")))
         :else (let
-          ((sx-handler
-             (fn (req-dict)
-               (er-http-resp-to-sx
-                 (er-apply-fun
-                   handler
-                   (list (er-http-req-of-sx req-dict)))))))
+          ((sx-handler (fn (req-dict) (er-http-resp-to-sx (er-apply-fun handler (list (er-http-req-of-sx req-dict)))))))
           (http-listen port sx-handler))))))
 
+;; Register everything at load time.
 (define
   er-register-builtin-bifs!
   (fn