fed-sx-m2: Step 1b — nx_kernel multi-actor gen_server calls + 9 tests

New gen_server exports add_actor/3, publish_to/2, log_tip_for/1,
actors/0, state_for/1, bucket_for/1, with_projections_for/2 —
each is a thin gen_server:call delegating to 1a's pure-functional
bucket API via fresh handle_call branches. Existing single-actor
calls (publish/1, log_tip/0, with_projections/1) route through
bucket 0 unchanged.

Per-actor mailbox sharding (one gen_server per bucket so distinct-
actor publishes don't serialise on a single mailbox) is forward-
looking — deferred to Step 4 where the per-actor HTTP routing makes
it actually load-bearing. Single-mailbox serialisation is fine for
Steps 1-3.

nx_kernel_multi.sh extended from 17 to 26 cases (gen_server load,
start_link bucket-0 seed, add_actor/3 dup detection, publish_to/2
per-actor isolation, interleaved publishes, no_actor error, state_for
+ with_projections_for round-trips). 134/134 across 12 nx_kernel-
adjacent + http suites. Erlang conformance 761/761 preserved.

plans/fed-sx-milestone-2.md

@@ -132,14 +132,18 @@ actors.
   the substrate; local `has_keyed`/`find_keyed`/`set_keyed`/
   `set_bucket` helpers handle the keyed-list ops.
   `next/tests/nx_kernel_multi.sh` 17/17.
-- [ ] **1b** — Multi-actor gen_server. `start_link/3` still works as
-  the single-actor entry; add `add_actor/3` (gen_server call,
-  bumps bucket), `publish_to/2(ActorId, Request)`, `log_tip_for/1`,
-  `actors/0`, `state_for/1`, `with_projections_for/2`. Existing
-  `publish/1`/`log_tip/0`/etc route through bucket-0 unchanged.
-  Concurrent publishes to distinct actors don't serialise across the
-  mailbox (multiple casts queued before each is processed). New tests
-  extend `nx_kernel_multi.sh` with 6-8 gen_server-mediated cases.
+- [x] **1b** — Multi-actor gen_server. `start_link/3` still seeds
+  bucket 0; new exports `add_actor/3`, `publish_to/2(ActorId,
+  Request)`, `log_tip_for/1`, `actors/0`, `state_for/1`,
+  `bucket_for/1`, `with_projections_for/2` delegate to the pure-
+  functional bucket APIs via fresh `handle_call` branches. Existing
+  `publish/1`/`log_tip/0`/`with_projections/1` route through bucket
+  0 unchanged. Per-actor mailbox concurrency (one gen_server per
+  bucket so distinct-actor publishes don't serialise) is forward-
+  looking — deferred to Step 4 (multi-actor HTTP routing) where it
+  actually pays off. `nx_kernel_multi.sh` extended with 9 gen_server
+  cases (26 total), every M1 nx_kernel-adjacent + http suite still
+  green (134 / 134 across 12 suites).
 
 **Acceptance:** `bash next/tests/nx_kernel_multi.sh` passes 12+ cases.
 
@@ -676,6 +680,19 @@ proceed.
 
 Newest first.
 
+- **2026-06-06** — Step 1b: gen_server multi-actor calls.
+  `nx_kernel` exports `add_actor/3`, `publish_to/2`, `log_tip_for/1`,
+  `actors/0`, `state_for/1`, `bucket_for/1`,
+  `with_projections_for/2` — each is a `gen_server:call` delegating
+  to the pure-functional bucket API from 1a. Existing single-actor
+  calls untouched. `nx_kernel_multi.sh` extended with 9 gen_server
+  cases (26 total); 134 / 134 across 12 nx_kernel-adjacent + http
+  suites. Conformance 761/761 preserved. Per-actor mailbox sharding
+  noted as forward-looking — current single gen_server serialises
+  publishes across actors, which is fine for Steps 1-3 (single-actor
+  HTTP endpoints) and is naturally untangled by Step 4's per-actor
+  routing.
+
 - **2026-06-06** — Step 1a: per-actor bucket refactor of `nx_kernel`.
   State shape now `[{actors, [{Id, Bucket}, …]}, {next_actor_seq, N}]`;
   added pure-functional multi-actor APIs (`new/0`, `add_actor/4`,