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.

next/README.md

@@ -122,17 +122,20 @@ These three gaps block the remaining unchecked deliverables:
    unchanged.
 
 3. **Dict ↔ proplist marshalling for `http:listen/2`** — **done 2026-06-05.**
-   `er-bif-http-listen` now marshals the native server's request dict
+   `er-bif-http-listen` marshals the native server's request dict
    (`{:method :path :query :headers :body}`) into the proplist shape
    `[{method, Bin}, {path, Bin}, {query, Bin}, {headers, [{Name, Value}]},
    {body, Bin}]` that `http_server:route/2` consumes, and converts the
    handler's response proplist back to `{:status :headers :body}` for the
-   native server to serialise. Helpers (`er-http-req-of-sx`,
-   `er-http-resp-to-sx`, `er-http-headers-of-sx`, `er-http-headers-to-sx`,
-   `er-mk-proplist`, `er-proplist-get`, `er-binary->string`,
-   `string->er-binary`) live alongside the BIF wrapper in
-   `lib/erlang/runtime.sx`. Verified by `next/tests/http_listen_bridge.sh`
-   (20 cases) including a `http_server:route/1` round-trip. Unblocks
+   native server to serialise. Helpers (`er-request-dict-to-proplist`,
+   `er-proplist-to-dict`, `er-of-sx-deep`, `er-to-sx-deep`,
+   `er-dict-to-header-proplist`, `er-proplist-fill!`) live alongside the
+   BIF wrapper in `lib/erlang/runtime.sx`. The BIF also spawns the handler
+   into a real Erlang process via `er-spawn-fun` + `er-sched-run-all!`
+   so `self()` / `gen_server:call` work inside route handlers (the kernel
+   and projection gen_servers reach the handler this way). Verified by
+   `next/tests/http_marshal.sh` and the live TCP smoke
+   `next/tests/http_server_tcp.sh` / `http_server_start.sh`. Unblocks
    `Step 8b-start` (TCP listener spawn) and the curl-driven 9a-tcp / 9b-tcp
    smoke tests.