fed-sx-m2: resolve Blockers #4 — kernel routes now work over real HTTP

Substrate fix: two-line change to lib/erlang/runtime.sx that lets
http-listen handler routes call gen_server:call without deadlocking.

  1. er-sched-step-alive!: pass :pending-args (when set) to the
     initial-fun call instead of always passing an empty list.
     Default behavior (no field) stays (list) — drop-in safe.

  2. er-bif-http-listen sx-handler: instead of er-apply-fun handler
     inline (which blows up on receive's er-suspend-marker because
     the connection thread has no scheduler step on its stack),
     create a real er-process with :initial-fun = handler and
     :pending-args = (list req-pl), then er-sched-run-all! to drain.
     Any receive (e.g. gen_server:call) suspends + resumes inside
     the SX scheduler frame the process owns. Read :exit-result
     for the response proplist; marshal back to SX dict.

Investigation arc (see plans/fed-sx-milestone-2.md Blockers #4 +
Progress log):
  - loops/fed-prims bf8d0bf2 diagnosed it as Erlang-substrate, not
    OCaml mutex (Pattern A wrong, Pattern B right but sketchy).
  - First Pattern B attempt failed: tried er-spawn-fun on a raw SX
    lambda, hit (er-fun? fv) gate. Connection-thread bisect
    pinpointed the exact line.
  - Real fix: use the existing er-fun (user's handler) directly,
    but feed it via :pending-args so step-alive's hardcoded
    (list) doesn't drop the request arg.

Acceptance:
  - new next/tests/smoke_kernel_route.sh: 6/6 over real HTTP
    (welcome /, /actors/alice, /actors/alice/outbox with
    gen_server-backed tip, /actors/alice/inbox, unknown-actor,
    via http_server:start(P, [{kernel, nx_kernel}])).
  - next/tests/http_server_tcp.sh: 5/5 (bumped wait_bound from
    30s to 180s — cold boot is slow under sibling-loop CPU load
    and the per-handler scheduler ramp adds a small margin).
  - Erlang conformance: 761/761.

Step 12's two-instance smoke test is now unblocked — its full
Follow / Accept / Note flow can layer on top of this kernel-route
surface. m2 plan updated.

Pre-existing httpc_request.sh flakiness ("Undefined symbol:
http-request" on the live-call epochs) reproduces WITHOUT this
change — see git stash A/B in the investigation. Unrelated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/erlang/runtime.sx

@@ -731,7 +731,10 @@
           0
           (if
             (= prev-k nil)
-            (er-apply-fun (er-proc-field pid :initial-fun) (list))
+            (er-apply-fun
+              (er-proc-field pid :initial-fun)
+              (let ((args (er-proc-field pid :pending-args)))
+                (cond (= args nil) (list) :else args)))
             (do (er-proc-set! pid :continuation nil) (prev-k nil)))))
       (let
         ((r (nth result-ref 0)))
@@ -1612,11 +1615,31 @@
           ;; 78eae9ef deleted them as dead because the BIF body
           ;; still referenced them — Blockers #1. This rewrite
           ;; threads through the live marshallers instead.)
+          ;; Run the handler as a SCHEDULED er-process so any
+          ;; `receive` (e.g. gen_server:call inside a kernel-aware
+          ;; route) suspends and resumes inside the SX scheduler.
+          ;; Without this, native http-listen invokes the handler
+          ;; closure on a fresh OCaml thread that has no scheduler
+          ;; frame, so the receive's er-suspend-marker propagates
+          ;; out and the connection writes nothing — the Blockers
+          ;; #4 deadlock the m2 loop observed.
+          ;;
+          ;; er-spawn-fun requires an er-fun (Erlang-AST-shaped
+          ;; dict); handler IS one (created by user `fun (Req) ->
+          ;; route(Req, Cfg) end`). To feed req-pl as the call
+          ;; argument we stash it on the process record's
+          ;; :pending-args field — er-sched-step-alive! reads it
+          ;; on first step (the alternative was a host-closure-to-
+          ;; er-fun wrapper, which needs AST construction).
           ((sx-handler
              (fn (req-dict)
                (let ((req-pl (er-request-dict-to-proplist req-dict)))
-                 (let ((resp-pl (er-apply-fun handler (list req-pl))))
-                   (er-proplist-to-dict resp-pl))))))
+                 (let ((proc (er-proc-new! (er-env-new))))
+                   (dict-set! proc :initial-fun handler)
+                   (dict-set! proc :pending-args (list req-pl))
+                   (er-sched-run-all!)
+                   (let ((resp-pl (er-proc-field (get proc :pid) :exit-result)))
+                     (er-proplist-to-dict resp-pl)))))))
           (http-listen port sx-handler))))))
 
 ;; httpc:request/4(Url, Method, Headers, Body) - BRIEFING-EXCEPTION: