fed-sx-m2: Step 1a — nx_kernel per-actor bucket refactor + 17 tests

State shape becomes [{actors, [{Id, Bucket}, ...]}, {next_actor_seq, N}]
with ActorBucket = [{key_spec, KS}, {actor_state, AS}, {log, L},
{projections, [Name]}, {next_published, N}]. Pure-functional multi-
actor APIs (new/0, add_actor/4, has_actor/2, actors/1, actor_count/1,
publish/3, per-actor accessors, with_actor_projections/3) join the
legacy single-actor accessors, which now read from the first bucket.
Every M1 test continues to pass via bootstrap:start/3 -> new/3 ->
first-bucket lookup.

Local has_keyed/find_keyed/set_keyed/set_bucket helpers cover the
keyed-list ops since lists:keymember/keyfind aren't registered in
this substrate.

next/tests/nx_kernel_multi.sh 17/17. M1 nx_kernel-adjacent suites
green (bootstrap_start 10/10, nx_kernel_server 11/11, http_publish
10/10, smoke_app_pure 12/12, http_post_format 13/13, http_publish_fold
10/10, http_marshal 10/10). Erlang conformance 761/761 preserved.

Blockers entry added for pre-existing http_server_tcp.sh 0/5
regression (78eae9ef left dead helper references in runtime.sx:1593) —
substrate-side, out of m2 scope, confirmed pre-existing by reverting
1a's changes and re-running.

next/README.md

@@ -43,7 +43,7 @@ next/
 | `bootstrap.erl`       | Genesis read/build/verify/load + one-call `start/3` kernel bring-up    |
 | `define_registry.erl` | Meta-projection fold for `Create{Define*}` → registry                  |
 | `sandbox.erl`         | `eval_pure/2,3` try/catch envelope for projection folds                |
-| `nx_kernel.erl`       | Long-lived runtime orchestrator (state + gen_server)                    |
+| `nx_kernel.erl`       | Long-lived runtime orchestrator; per-actor bucketed state (m2 Step 1a)  |
 | `http_server.erl`     | route/1,2 + format-aware GET + POST + Accept header content negotiation |
 
 ## Genesis bundle