fed-sx-m2: Step 5d — inbox handler wires the ingestion chain

POST /actors/<id>/inbox is now special-cased in route/2 (next to
POST /activity) so the body + Cfg reach the new handle_inbox_post/3
handler.

Wire format: body = term_codec:encode(SignedActivity); the receiver
decodes into the activity proplist and runs the chain.

handle_inbox_post/3 orchestration:
  1. kernel_has_actor(field(kernel, Cfg), TargetId)  -> 404 if missing
  2. decode_activity(Body)                           -> 422 on bad shape
  3. envelope:get_field(actor, Activity)             -> 422 if no peer id
  4. resolve_peer_as(PeerId, Cfg)                    -> 401 if unknown
  5. nx_kernel:inbox_state_for(TargetAtom)           -> 404 belt-and-braces
  6. pipeline:validate_inbound(Activity, PeerAS, InboxLog)
       ok                     -> nx_kernel:append_inbox + 202
       {error, bad_signature} -> 401
       {error, no_signature}  -> 401
       {error, _}             -> 422

resolve_peer_as/2 supports three Cfg paths in priority order:
  {peer_as,        [{PeerId, AS}, ...]}   pure-fn pre-populated map
  {peer_actors,    AtomName}              peer_actors gen_server cache
  {peer_fetch_fn,  fun/1}                 fallback on srv cache miss
Empty Cfg returns {error, no_peer_resolver} -> 401.

v1 actor_post/1 4a stub deleted; M1 actor_inbox_post_response/0
kept for response composition.

Projection broadcast on inbox success intentionally deferred to a
follow-up sub-deliverable.

inbox.sh 11/11 (acceptance suite for the basic chain):
  - happy path -> 202
  - inbox tip advances; outbox tip unchanged (per-actor bucket
    independence carried through from Step 5a)
  - empty / garbage body -> 422
  - unknown peer -> 401
  - bad peer-AS keys -> 401
  - replay (same activity twice) -> 422 on second
  - unknown target actor -> 404
  - two distinct activities -> tip = 2

inbox_peer_resolution.sh 6/6 (Cfg resolution variants):
  - peer_actors gen_server hit -> 202
  - FetchFn fallback -> 202
  - FetchFn error -> 401
  - FetchFn caches into peer_actors (peers_srv shows [bob] after)
  - No resolver -> 401

Tests split into two files because each epoch's kernel start_link
+ outbox construct + term_codec encode is expensive and a single
suite hits the wall-clock budget.

http_server.erl is now 1181 lines. erlang-load-module on this port
scales superlinearly with function count, so eight http_*.sh tests'
internal sx_server timeout bumped 60s -> 360s (http_route,
http_actors, http_accept, http_capabilities, http_capabilities_format,
http_content_type, http_artifacts, http_projections).

Conformance 761/761.

next/tests/http_multi_actor.sh

@@ -52,6 +52,8 @@ cat > "$TMPFILE" <<'EPOCHS'
 (eval "(get (erlang-load-module (file-read \"next/kernel/outbox.erl\")) :name)")
 (epoch 8)
 (eval "(get (erlang-load-module (file-read \"next/kernel/nx_kernel.erl\")) :name)")
+(epoch 9)
+(eval "(get (erlang-load-module (file-read \"next/kernel/term_codec.erl\")) :name)")
 
 ;; split_first_slash sanity
 (epoch 10)
@@ -81,9 +83,11 @@ cat > "$TMPFILE" <<'EPOCHS'
 (epoch 24)
 (eval "(get (erlang-eval-ast \"Req = [{method, <<71,69,84>>}, {path, <<47,97,99,116,111,114,115,47,97,108,105,99,101,47,102,111,108,108,111,119,105,110,103>>}, {headers, []}, {body, <<>>}], R = http_server:route(Req), case R of [{status, 200}, _, {body, B}] -> http_server:match_prefix(<<102,111,108,108,111,119,105,110,103,58>>, B) =/= nomatch; _ -> false end\") :name)")
 
-;; POST /actors/alice/inbox returns 202 with "accepted"
+;; POST /actors/alice/inbox with empty body -> 422 (Step 5d
+;; expects a term_codec-encoded signed activity; empty body fails
+;; decoding before sig check runs).
 (epoch 25)
-(eval "(get (erlang-eval-ast \"Req = [{method, <<80,79,83,84>>}, {path, <<47,97,99,116,111,114,115,47,97,108,105,99,101,47,105,110,98,111,120>>}, {headers, []}, {body, <<>>}], R = http_server:route(Req), case R of [{status, 202}, _, {body, B}] -> http_server:match_prefix(<<97,99,99,101,112,116,101,100>>, B) =/= nomatch; _ -> false end\") :name)")
+(eval "(get (erlang-eval-ast \"Req = [{method, <<80,79,83,84>>}, {path, <<47,97,99,116,111,114,115,47,97,108,105,99,101,47,105,110,98,111,120>>}, {headers, []}, {body, <<>>}], R = http_server:route(Req), case R of [{status, 422}, _, _] -> true; _ -> false end\") :name)")
 
 ;; GET /actors/alice/unknown returns 404
 (epoch 26)
@@ -256,7 +260,7 @@ check 21  "GET /actors/<id>/outbox stub"      "true"
 check 22  "GET /actors/<id>/inbox stub"       "true"
 check 23  "GET /actors/<id>/followers stub"   "true"
 check 24  "GET /actors/<id>/following stub"   "true"
-check 25  "POST /actors/<id>/inbox -> 202"    "true"
+check 25  "POST inbox empty body -> 422"      "true"
 check 26  "GET /actors/<id>/<bad> -> 404"     "true"
 check 27  "POST /actors/<id>/<bad> -> 404"    "true"
 check 28  "GET /actors/ (empty) -> 404"       "true"