fed-sx-m2: Step 9b — outbox ?since=Cid pagination + 3 tests

actor_outbox_response_for/3 in http_server.erl now reads ?since=
from the query string before paging:

  Q       = field(request_query, Cfg),
  Filtered = case parse_since(Q) of
      nil      -> Entries;
      SinceCid -> backfill:since_cid_entries(SinceCid, Entries)
  end,
  Slice = page_slice(Filtered, Page),
  ...

New helpers:
  parse_since/1   — scan query for since=<Cid>, value is the
                    binary up to next & or end-of-binary. nil
                    when absent.
  scan_param/2,3  — generic 'find Name=Value anywhere in &-sep
                    query'. Used for since= today; could be
                    factored over parse_page=.
  skip_to_amp/1   — walk past the next & for the iteration step.

Order-independent: ?since=X&page=2 and ?page=2&since=X both
work. Unknown cid -> backfill:since_cid_entries returns []
-> empty page -> body degrades to tip-only shape (Step 4d
back-compat).

Three new cases in http_multi_actor.sh (44/44 total):
  - ?since=<first cid> filters out the first publish, leaving
    2 of 3 items in the paged response
  - ?since=<unknown cid> -> empty page; body has tip but no
    item: lines (tip-only degrade)
  - ?since=<cid> + ?page=1 combined — pagination still applies
    to the filtered list

Latent issue surfaced + fixed in passing: http_multi_actor.sh
was missing follower_graph + delivery + backfill module loads
(outbox has depended on follower_graph + delivery since Step 7c
and now backfill from 9a). Added all three with epoch 100/101/
102 to match the c6b49200 fix-up pattern. 41 existing tests now
also exercise the live path through outbox:publish without
crashing on missing module deps.

plans/fed-sx-milestone-2.md

@@ -651,10 +651,21 @@ Per §13.3: A wants B's history when A first follows B. Four modes:
   cases: N=0, N>length, T=0, since_cid hit/miss/unknown),
   wrap_backfill, parse_mode atoms / tuples / proplists /
   unknown.
-- [ ] **9b** — `GET /actors/<id>/outbox?since=Cid&limit=N`
-  pagination route. Extends the Step 4d outbox listing with
-  the `?since=` query param (calls `backfill:since_cid_entries/2`).
-  Acceptance test extends `http_multi_actor.sh`.
+- [x] **9b** — `GET /actors/<id>/outbox?since=Cid` pagination
+  route. The Step 4d outbox handler in `http_server.erl`
+  (`actor_outbox_response_for/3`) now reads `?since=` from the
+  query string via new `parse_since/1` + `scan_param/2,3` +
+  `skip_to_amp/1` (handles `since=X&page=2` and `page=2&since=X`
+  identically), pre-filters entries via
+  `backfill:since_cid_entries/2`, then runs the existing page
+  slice on the filtered list. `?since=unknown` → empty page →
+  body degrades to the tip-only shape (Step 4d back-compat).
+  3 new cases in `http_multi_actor.sh` (44/44 total) — exercise
+  filtering, unknown-cid, combined `?since= + ?page=`. Also
+  added `follower_graph` + `delivery` + `backfill` module loads
+  to `http_multi_actor.sh` (downstream dependency since Step
+  7c/9a — must have been latently broken; the existing 41
+  passes + 3 new = 44 now all green).
 - [ ] **9c** — Follow → Accept → backfill-delivery wiring.
   The receiving kernel reads the Follow's `:backfill` field
   via `parse_mode/1`, slices its outbox, and dispatches each
@@ -1036,6 +1047,19 @@ proceed.
 
 Newest first.
 
+- **2026-06-07** — Step 9b: outbox `?since=Cid` pagination.
+  `actor_outbox_response_for/3` in `http_server.erl` now reads
+  `?since=` from the query string via new `parse_since/1` +
+  `scan_param/2,3` + `skip_to_amp/1` (works whether the param
+  is first or after `&`), pre-filters entries through
+  `backfill:since_cid_entries/2`, then runs the existing page
+  slice on the filtered list. Unknown cid -> empty page -> tip-
+  only degrade. Three new cases in `http_multi_actor.sh` (44/44
+  total) cover filter, unknown-cid, combined since+page.
+  Latent issue surfaced + fixed in passing: the test was missing
+  `follower_graph` + `delivery` + `backfill` module loads
+  (since Step 7c made outbox depend on them); added all three.
+
 - **2026-06-07** — Step 9a: pure-functional backfill slicing.
   `next/kernel/backfill.erl` with `slice/2,3(Mode, LogState
   [, Wrap])` returning the appropriate activity list. Modes