fed-sx-m2: Step 4a — per-actor HTTP sub-paths + 17 tests

Per design §16.1 each actor has /outbox /inbox /followers /following
sub-paths. New split_first_slash/1 helper lets the GET /actors/...
dispatch arm fan out on the sub-segment:

  GET  /actors/<id>            actor doc (M1 — unchanged)
  GET  /actors/<id>/outbox     outbox stub (4a)
  GET  /actors/<id>/inbox      inbox stub (4a)
  GET  /actors/<id>/followers  follower stub (4a)
  GET  /actors/<id>/following  following stub (4a)
  POST /actors/<id>/inbox      202 Accepted stub (4a; Step 5 real)

Four new content-negotiated response functions mirror the existing
actor_doc_response_for/2 shape (text / json / activity_json / sx
variants):

  actor_outbox_response_for/2
  actor_inbox_get_response_for/2
  actor_followers_response_for/2
  actor_following_response_for/2

POST returns 202 via new accepted_response/1 +
actor_inbox_post_response/0.

Unknown sub-paths under /actors/<id>/ return 404. Bare /actors/<id>
preserves the M1 actor-doc arm so http_route + http_post_format
regression suites stay green.

4b-4e (token map, route/3 kernel access, per-actor outbox listing
from log entries, real inbox pipeline) layer on top of this dispatch
in subsequent iterations.

17/17 in next/tests/http_multi_actor.sh covering:
  - split_first_slash sanity (no slash / id+sub / trailing slash)
  - all four GET sub-paths return 200 with stub bodies
  - POST inbox returns 202 + 'accepted'
  - unknown sub-paths return 404 (GET and POST)
  - empty /actors/ returns 404
  - body carries the actor id
  - content negotiation: outbox JSON, inbox SX, followers JSON

Conformance 761/761. 120/120 across 10 Step-4-adjacent suites
(http_route, http_publish, http_post_format, http_marshal,
http_publish_fold, http_listen_bif, http_server_start,
nx_kernel_multi, actor_state_pure, bootstrap_start).

next/kernel/http_server.erl

@@ -16,7 +16,11 @@
          content_type_for/1, ok_response/2,
          cid_response_for/2, post_activity_response_for/1,
          actor_doc_response_for/2, artifact_response_for/2,
-         projection_response_for/2, projections_list_response_for/1]).
+         projection_response_for/2, projections_list_response_for/1,
+         actor_outbox_response_for/2, actor_inbox_get_response_for/2,
+         actor_followers_response_for/2, actor_following_response_for/2,
+         actor_inbox_post_response/0, accepted_response/1,
+         split_first_slash/1]).
 
 %% HTTP request router per design §16.1.
 %%
@@ -94,11 +98,11 @@ dispatch(<<71, 69, 84>>,
 %% prefix clause because the bare path has no trailing slash.
 dispatch(<<71, 69, 84>>, <<47,112,114,111,106,101,99,116,105,111,110,115>>, F) ->
     projections_list_response_for(F);
-%% GET /actors/{id} or /artifacts/{cid} or /projections/{name}
+%% GET /actors/{id}[/sub] or /artifacts/{cid} or /projections/{name}
 dispatch(<<71, 69, 84>>, Path, F) ->
     case match_prefix(actors_prefix(), Path) of
-        {ok, Id} when byte_size(Id) > 0 ->
-            actor_doc_response_for(Id, F);
+        {ok, Rest} when byte_size(Rest) > 0 ->
+            actor_get(Rest, F);
         _ ->
             case match_prefix(artifacts_prefix(), Path) of
                 {ok, Cid} when byte_size(Cid) > 0 ->
@@ -112,9 +116,71 @@ dispatch(<<71, 69, 84>>, Path, F) ->
                     end
             end
     end;
+%% POST /actors/{id}/inbox — peer-side delivery (Step 4a returns
+%% 202 Accepted stub; Step 5 lands the real ingestion pipeline).
+dispatch(<<80, 79, 83, 84>>, Path, _F) ->
+    case match_prefix(actors_prefix(), Path) of
+        {ok, Rest} when byte_size(Rest) > 0 ->
+            actor_post(Rest);
+        _ ->
+            not_found_response()
+    end;
 dispatch(_, _, _) ->
     not_found_response().
 
+%% actor_get/2 — Rest is the part after "/actors/". If it has no
+%% inner slash, it's the bare actor doc. Otherwise dispatch on the
+%% sub-segment.
+
+actor_get(Rest, F) ->
+    case split_first_slash(Rest) of
+        {Id, <<>>}    -> actor_doc_response_for(Id, F);
+        {Id, Sub}     -> actor_subresource_get(Id, Sub, F);
+        Id            -> actor_doc_response_for(Id, F)
+    end.
+
+%% 111 117 116 98 111 120 = "outbox"
+actor_subresource_get(Id, <<111,117,116,98,111,120>>, F) ->
+    actor_outbox_response_for(Id, F);
+%% 105 110 98 111 120 = "inbox"
+actor_subresource_get(Id, <<105,110,98,111,120>>, F) ->
+    actor_inbox_get_response_for(Id, F);
+%% 102 111 108 108 111 119 101 114 115 = "followers"
+actor_subresource_get(Id, <<102,111,108,108,111,119,101,114,115>>, F) ->
+    actor_followers_response_for(Id, F);
+%% 102 111 108 108 111 119 105 110 103 = "following"
+actor_subresource_get(Id, <<102,111,108,108,111,119,105,110,103>>, F) ->
+    actor_following_response_for(Id, F);
+actor_subresource_get(_, _, _) ->
+    not_found_response().
+
+actor_post(Rest) ->
+    case split_first_slash(Rest) of
+        {_Id, <<105,110,98,111,120>>} ->
+            actor_inbox_post_response();
+        _ ->
+            not_found_response()
+    end.
+
+%% split_first_slash/1 — split a binary on the first slash. Returns
+%% {Before, After} where After omits the slash itself. If no slash
+%% is present, returns just Before. 47 = "/".
+%%
+%%   <<"alice">>           -> <<"alice">>
+%%   <<"alice/">>          -> {<<"alice">>, <<>>}
+%%   <<"alice/inbox">>     -> {<<"alice">>, <<"inbox">>}
+%%   <<"alice/inbox/x">>   -> {<<"alice">>, <<"inbox/x">>}
+
+split_first_slash(Bin) ->
+    split_first_slash(Bin, <<>>).
+
+split_first_slash(<<>>, Acc) ->
+    Acc;
+split_first_slash(<<47, Rest/binary>>, Acc) ->
+    {Acc, Rest};
+split_first_slash(<<B, Rest/binary>>, Acc) ->
+    split_first_slash(Rest, <<Acc/binary, B>>).
+
 %% "fed-sx kernel m1\n" — 17 bytes, hand-spelled.
 %%  f  e  d  -  s  x   _   k   e   r   n   e   l   _   m   1   \n
 welcome_body() ->
@@ -538,6 +604,105 @@ actor_doc_response_for(Id, cbor) ->
 actor_doc_response_for(Id, _) ->
     actor_doc_response(Id).
 
+%% ── Step 4a: per-actor sub-resource stubs ──────────────────────
+%% Per design §16.1 each actor has /outbox /inbox /followers
+%% /following routes. v1 returns text-stub bodies so route resolution
+%% can be tested end-to-end; real serialisation of per-actor outbox
+%% listings (Step 4d) + follower-graph projection bodies (Step 6+)
+%% layer on top of these dispatch arms.
+
+%% "outbox: " — 8 bytes
+actor_outbox_response_for(Id, text) ->
+    Pre = <<111,117,116,98,111,120,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>);
+actor_outbox_response_for(Id, json) ->
+    Pre = <<123,34,111,117,116,98,111,120,34,58,34>>,  % '{"outbox":"'
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, json);
+actor_outbox_response_for(Id, activity_json) ->
+    Pre = <<123,34,111,117,116,98,111,120,34,58,34>>,
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, activity_json);
+actor_outbox_response_for(Id, sx) ->
+    Pre = <<40,111,117,116,98,111,120,32,34>>,         % '(outbox "'
+    Suf = <<34,41,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, sx);
+actor_outbox_response_for(Id, _) ->
+    Pre = <<111,117,116,98,111,120,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>).
+
+%% "inbox: " — 7 bytes
+actor_inbox_get_response_for(Id, text) ->
+    Pre = <<105,110,98,111,120,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>);
+actor_inbox_get_response_for(Id, json) ->
+    Pre = <<123,34,105,110,98,111,120,34,58,34>>,      % '{"inbox":"'
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, json);
+actor_inbox_get_response_for(Id, activity_json) ->
+    Pre = <<123,34,105,110,98,111,120,34,58,34>>,
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, activity_json);
+actor_inbox_get_response_for(Id, sx) ->
+    Pre = <<40,105,110,98,111,120,32,34>>,             % '(inbox "'
+    Suf = <<34,41,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, sx);
+actor_inbox_get_response_for(Id, _) ->
+    Pre = <<105,110,98,111,120,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>).
+
+%% "followers: " — 11 bytes
+actor_followers_response_for(Id, text) ->
+    Pre = <<102,111,108,108,111,119,101,114,115,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>);
+actor_followers_response_for(Id, json) ->
+    Pre = <<123,34,102,111,108,108,111,119,101,114,115,34,58,34>>,
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, json);
+actor_followers_response_for(Id, activity_json) ->
+    Pre = <<123,34,102,111,108,108,111,119,101,114,115,34,58,34>>,
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, activity_json);
+actor_followers_response_for(Id, sx) ->
+    Pre = <<40,102,111,108,108,111,119,101,114,115,32,34>>,
+    Suf = <<34,41,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, sx);
+actor_followers_response_for(Id, _) ->
+    Pre = <<102,111,108,108,111,119,101,114,115,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>).
+
+%% "following: " — 11 bytes
+actor_following_response_for(Id, text) ->
+    Pre = <<102,111,108,108,111,119,105,110,103,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>);
+actor_following_response_for(Id, json) ->
+    Pre = <<123,34,102,111,108,108,111,119,105,110,103,34,58,34>>,
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, json);
+actor_following_response_for(Id, activity_json) ->
+    Pre = <<123,34,102,111,108,108,111,119,105,110,103,34,58,34>>,
+    Suf = <<34,125,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, activity_json);
+actor_following_response_for(Id, sx) ->
+    Pre = <<40,102,111,108,108,111,119,105,110,103,32,34>>,
+    Suf = <<34,41,10>>,
+    ok_response(<<Pre/binary, Id/binary, Suf/binary>>, sx);
+actor_following_response_for(Id, _) ->
+    Pre = <<102,111,108,108,111,119,105,110,103,58,32>>,
+    ok_response(<<Pre/binary, Id/binary, 10>>).
+
+%% POST /actors/<id>/inbox stub — 202 Accepted with body "accepted\n".
+%% Real ingestion pipeline (sig verify + envelope:get_field + log
+%% append on the receiving actor's inbox bucket) lands in Step 5.
+
+actor_inbox_post_response() ->
+    %% "accepted\n" — 9 bytes
+    Body = <<97,99,99,101,112,116,101,100,10>>,
+    accepted_response(Body).
+
+accepted_response(Body) ->
+    [{status, 202}, {headers, []}, {body, Body}].
+
 %% artifact_response — text body `artifact: <cid>\n`.
 
 artifact_response_for(Cid, text) ->