fed-sx-m2: Step 4c — route/3 with kernel access + 8 tests

http_server:route/3(Req, Cfg, Kernel) is the new extended entry
point: folds the kernel reference (typically the registered
nx_kernel atom) into Cfg as {kernel, Kernel}. route/2 is
unchanged and stays the M1 surface.

The dispatch chain gained Cfg threading all the way down:
  dispatch/3 -> dispatch/4 (M, P, F, Cfg)
  actor_get/2 -> actor_get/3 (Rest, F, Cfg)
  actor_subresource_get/3 -> /4 (Id, Sub, F, Cfg)

actor_outbox_response_for/3 (new) reads :kernel from Cfg and,
when the kernel atom is registered AND the actor exists, renders
'tip: <N>' alongside the actor id in text / JSON / SX content-
negotiated bodies. Unknown actors or unregistered kernels fall
back to the 4a stub.

Inbox / followers / following handlers accept Cfg but ignore it
for now — they layer real state lookup in 4d/4e/Step 5+.

Substrate gotcha logged in the Progress log: try/of/catch around
gen_server:call(nx_kernel, _) deadlocks in this port's scheduler
(probably the catch frame's mask defers reply delivery). The
live kernel_log_tip/2 helper does a bare call + integer guard
instead. nx_kernel_multi.sh already proves bare gen_server:call
into the same kernel works correctly.

8 new cases in next/tests/http_multi_actor.sh (33/33 total):
  - route/3 with registered kernel: outbox body includes tip=0
  - tip advances after POST publish through route/3 + token map
  - unknown actor (ghost) falls back to 4a stub (no tip:)
  - unregistered kernel ref falls back to stub
  - JSON Accept renders {"outbox":"alice","tip":0}
  - SX Accept renders (outbox "alice" :tip 0)
  - Bob's outbox tip stays 0 while Alice publishes (per-actor)
  - route/2 path unchanged: no tip field in body

Conformance 761/761. 121/121 across 10 Step-4-adjacent suites
(http_multi_actor, http_route, http_publish, http_post_format,
http_marshal, http_publish_fold, http_listen_bif, http_server_start,
nx_kernel_multi, bootstrap_start, actor_lifecycle).

plans/fed-sx-milestone-2.md

@@ -285,10 +285,21 @@ token; the token now maps to an `:actor_id` rather than a fixed `alice`.
   No-kernel auth-only path unchanged. The dead M1
   `expected_token/1` helper is gone. 8 new cases in
   `http_multi_actor.sh` (25/25 total).
-- [ ] **4c** — `http_server:route/3(Req, Cfg, Kernel)` — the Cfg
-  carries opaque `:kernel` reference (or accepts the registered
-  `nx_kernel` atom) so per-actor handlers can call
-  `nx_kernel:state_for/1`, `actor_log_state/2`, projections etc.
+- [x] **4c** — `http_server:route/3(Req, Cfg, Kernel)` is sugar
+  that folds the Kernel reference (typically the registered
+  `nx_kernel` atom) into Cfg as `{kernel, Kernel}`. The dispatch
+  chain gained a Cfg arg threaded all the way to per-actor
+  sub-resource handlers (`dispatch/3` → `dispatch/4`, `actor_get/2`
+  → `actor_get/3`, `actor_subresource_get/3` → /4). The outbox
+  sub-resource handler now reads `:kernel` and, when the actor
+  exists in the kernel, renders `tip: <N>` in text / JSON / SX
+  variants — proving the plumbing works end-to-end. Unknown
+  actors or unregistered kernels fall back to the 4a stub.
+  `try`/`of`/`catch` around `gen_server:call` deadlocks in this
+  port's scheduler (probably the catch-frame mask defers reply
+  delivery); the live handler does a bare `nx_kernel:log_tip_for/1`
+  + integer guard instead. 8 new cases in `http_multi_actor.sh`
+  (33/33 total).
 - [ ] **4d** — Per-actor outbox listing reads from the named
   bucket's log entries via `nx_kernel:actor_log_state/2`, content-
   negotiates as today (text / json / sx). `?page=N` pagination
@@ -738,6 +749,22 @@ proceed.
 
 Newest first.
 
+- **2026-06-06** — Step 4c: route/3 with kernel access.
+  `http_server:route/3(Req, Cfg, Kernel)` folds the kernel
+  reference into Cfg as `{kernel, _}`. Dispatch chain refactored
+  to thread Cfg through to per-actor sub-resource handlers.
+  Outbox handler reads `:kernel` and renders `tip: <N>` (in
+  text / JSON / SX content-negotiated variants) when the actor
+  exists; falls back to the 4a stub otherwise. Substrate quirk
+  found: `try`/`of`/`catch` around `gen_server:call` deadlocks
+  in this port's scheduler — bare call + integer guard works.
+  Inbox / followers / following handlers accept Cfg but ignore
+  it; real state lookup lands in 4d/4e/Step 5+. 8 new cases in
+  `http_multi_actor.sh` (33/33 total). Conformance 761/761.
+  121/121 across 10 Step-4-adjacent suites. **Gotcha** noted
+  for future iterations: avoid try/catch wrapping gen_server
+  calls in this substrate.
+
 - **2026-06-06** — Step 4b: token -> ActorId map. Cfg's `:tokens`
   proplist (`[{Token, ActorId}, ...]`) maps bearer tokens to
   per-actor publishers. `handle_post_activity` threads the