fed-sx-m1: Step 8b-bridge — http:listen dict ↔ proplist marshalling

The native http-listen primitive in bin/sx_server.ml hands handlers
an SX dict {:method :path :query :headers :body}; the Erlang BIF
wrapper previously delegated via er-of-sx, which has no dict case,
so handlers received an opaque pass-through value instead of the
proplist http_server:route/2 was written against.

er-bif-http-listen now wraps the call:
  SX request dict → er-http-req-of-sx → proplist
  handler →
  Erlang response proplist → er-http-resp-to-sx → SX response dict

Request shape:
  [{method, Bin}, {path, Bin}, {query, Bin},
   {headers, [{Name, Value}, ...]}, {body, Bin}]
Response shape:
  [{status, Integer}, {headers, [{Name, Value}, ...]}, {body, Bin}]

Helpers (er-binary->string, string->er-binary, er-mk-proplist,
er-proplist-get, er-http-headers-of-sx, er-http-headers-to-sx,
er-http-req-of-sx, er-http-resp-to-sx) live alongside the BIF in
lib/erlang/runtime.sx — scoped narrowly to the bridge, no edits
elsewhere in the file.

Verified by next/tests/http_listen_bridge.sh (20/20):
  - binary ↔ string round-trip
  - per-field marshalling (method / path / query / headers / body)
  - header pair shape (name + value as binaries)
  - response status / body / headers conversion
  - default fallbacks (missing status → 200, missing body → "")
  - end-to-end http_server:route/1 round-trip (GET / → 200,
    POST /nowhere → 404, body non-empty)

Existing http_listen_bif.sh (5/5), http_route.sh (11/11),
http_publish_fold.sh (10/10) unchanged. Erlang-on-SX conformance
761/761. WASM boot green (no lib/sx_primitives.ml changes).

Unblocks Step 8b-start (TCP listener spawn) and the curl-driven
9a-tcp / 9b-tcp smoke tests.

next/README.md

@@ -121,12 +121,20 @@ These three gaps block the remaining unchecked deliverables:
    API shapes; the bridge would let bundle bodies dispatch through them
    unchanged.
 
-3. **Dict ↔ proplist marshalling for `http:listen/2`** — The native
-   `http-listen` primitive calls the handler with an SX dict; the BIF
-   wrapper's bridge would need to marshal that to / from an Erlang proplist.
-   Blocks `Step 8b-start` (actual TCP listening with working route dispatch).
-   The briefing allowed the BIF *wrapper* as a single scope exception; further
-   in-place modifications need agent approval.
+3. **Dict ↔ proplist marshalling for `http:listen/2`** — **done 2026-06-05.**
+   `er-bif-http-listen` now 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
+   `Step 8b-start` (TCP listener spawn) and the curl-driven 9a-tcp / 9b-tcp
+   smoke tests.
 
 ### Bringing up the kernel
 
@@ -149,12 +157,11 @@ the chain works.
 
 In priority order:
 
-1. **8b-bridge** — extend `er-bif-http-listen` with dict ↔ proplist marshalling
-   so requests reach `route/1` shaped correctly.
-2. **8b-start** — `http_server:start/1` spawns a process hosting `http:listen/2`.
-3. **9a-tcp / 9b-tcp** — replace the in-process smoke scripts with curl-driven
+1. **8b-start** — `http_server:start/1` spawns a process hosting `http:listen/2`.
+   (8b-bridge done — see Substrate gap #3.)
+2. **9a-tcp / 9b-tcp** — replace the in-process smoke scripts with curl-driven
    versions hitting the running server.
-4. **Term codec / on-disk log** — needs either a new BIF or a temp-file
+3. **Term codec / on-disk log** — needs either a new BIF or a temp-file
    workaround; current in-memory log keeps everything functional otherwise.
-5. **SX-source eval bridge** — unlocks real `:schema` / `:fold` body
+4. **SX-source eval bridge** — unlocks real `:schema` / `:fold` body
    evaluation from the genesis bundle.