fed-sx-m2: Step 4e — scope-boundary tick, no code change

POST /actors/<id>/inbox stays the 4a 202 'accepted' stub through
all of 4a-4d. The real inbound pipeline (peer sig verify + inbox-
bucket append + projection broadcast) is Step 5's whole topic, so
4e is closed as a deliberate scope boundary — no code change.

Step 4 fully closed (4a per-actor sub-paths, 4b token map,
4c route/3 + kernel access, 4d outbox listing + pagination, 4e
inbox-stays-stub).

plans/fed-sx-milestone-2.md

@@ -313,9 +313,12 @@ token; the token now maps to an `:actor_id` rather than a fixed `alice`.
   `fun F(...) -> ... F(...) end` not supported; `binary:matches/2`
   and `lists:foreach/2` not registered — tests prove behaviour
   via `match_prefix` substring checks rather than counting.
-- [ ] **4e** — POST /actors/<id>/inbox stays a 202 stub for 4a-4d.
-  Step 5 lands the real ingestion pipeline (sig verify + inbox-
-  bucket append + projection broadcast).
+- [x] **4e** — POST /actors/<id>/inbox stays the 4a 202 stub
+  through 4a-4d; the real ingestion pipeline (sig verify + inbox-
+  bucket append + projection broadcast) is Step 5's whole topic.
+  No code change for this checkbox — it's a deliberate scope
+  boundary so 4d's listing semantics land cleanly before
+  inbound traffic shapes the same per-actor URLs.
 
 **Acceptance:** `bash next/tests/http_multi_actor.sh` passes 14+ cases.