next: the real durable-execution KERNEL SERVICE (host_kernel) — RA-live substrate

Promotes the persistent-kernel spike into a real service. next/kernel/host_kernel.erl: boots
flow_store, registers named behavior flows (blog_digest), then blocks in http:listen so the
er-scheduler + gen_server stay alive across requests. Parameterised flow routes (paths matched by
byte prefix — binary =:= is buggy): GET /flow/start/<category> starts the flow with that category and
returns '<InstanceId>:<status>' (suspended|done); GET /flow/resume/<id> resumes that instance. Path
plumbing (starts_with / last_seg / field) is byte-level for portability.

next/kernel/serve.sh: the persistent service launcher (container entrypoint / local) — loads the
runtime + next/flow + the kernel, then host_kernel:start(); sleep infinity holds stdin so
the listener serves forever. next/tests/host_kernel.sh: drives it over HTTP — 4/4: newsletter →
instance 1 SUSPENDED, urgent → 2 DONE, draft → 3 DONE (skipped), resume 1 in a SEPARATE request →
DONE (durable state persists across requests). serve.sh launcher verified live (bind + start +
resume).

This is the RA-live substrate: a working durable-execution service the host drives over HTTP.
Remaining for RA-live: deploy it (a container/placement), point host/ra.sx's real-eval at it (POST
/flow instead of in-process erlang-eval-ast), route a durable binding to RA. TA-live adds inbox/
outbox routes on the same kernel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

next/kernel/host_kernel.erl

@@ -0,0 +1,71 @@
+%% next/kernel/host_kernel.erl — the persistent DURABLE-EXECUTION kernel the host talks to (RA-live).
+%% A long-lived next/ service: boots flow_store + registers named behavior flows, then blocks in
+%% http:listen so the er-scheduler (and flow_store's gen_server) stay alive across requests. The host
+%% drives durable flows over HTTP; instances persist between requests (spike-proven).
+%%
+%% Routes (path-parameterised — binary =:= is buggy in this port, so paths are matched by byte
+%% prefix, not equality):
+%;;   GET /flow/start/<category>  -> start blog_digest with that category -> "<InstanceId>:<status>"
+%%                                  (status = suspended | done)
+%%   GET /flow/resume/<id>       -> resume instance <id> (morning timer) -> "resume:<status>"
+%% This is the substrate for RA-live; TA-live adds inbox/outbox routes on the same kernel.
+-module(host_kernel).
+-export([start/1]).
+
+start(Port) ->
+    flow_store:start_link(),
+    FF = fun (_) -> [f1, f2, f3] end,
+    flow_store:register_flow(blog_digest, blog_publish_digest:build([{fetch_followers, FF}])),
+    http:listen(Port, fun (Req) -> route(Req) end).
+
+route(Req) ->
+    [{status, 200}, {headers, []}, {body, respond(field(path, Req))}].
+
+respond(P) ->
+    case starts_with(P, [47,102,108,111,119,47,115,116,97,114,116,47]) of        % "/flow/start/"
+        true -> do_start(last_seg(P));
+        false ->
+            case starts_with(P, [47,102,108,111,119,47,114,101,115,117,109,101,47]) of  % "/flow/resume/"
+                true  -> do_resume(last_seg(P));
+                false -> <<"path:unknown">>
+            end
+    end.
+
+%% start blog_digest with the path's <category> -> "<Id>:suspended" | "<Id>:done"
+do_start(CatChars) ->
+    Cat = list_to_atom(CatChars),
+    Env = [{activity, [{type, create}, {actor, site}, {id, <<110,49>>},
+                       {object, [{type, article}, {category, Cat}]}]},
+           {actor, site}],
+    case flow_store:start(blog_digest, Env) of
+        {ok, Id, {flow_suspended, _}} -> id_status(Id, [115,117,115,112,101,110,100,101,100]);  % "suspended"
+        {ok, Id, {flow_done, _}}      -> id_status(Id, [100,111,110,101]);                       % "done"
+        _                             -> <<"start:other">>
+    end.
+
+%% resume instance <id> -> "resume:done" | "resume:other"
+do_resume(IdChars) ->
+    case flow_store:resume(list_to_integer(IdChars), morning_ts) of
+        {ok, {flow_done, _}} -> <<"resume:done">>;
+        {flow_done, _}       -> <<"resume:done">>;
+        _                    -> <<"resume:other">>
+    end.
+
+%% "<Id>:<statusChars>" as a response binary
+id_status(Id, StatusChars) ->
+    list_to_binary(integer_to_list(Id) ++ [58] ++ StatusChars).   % ++ ":" ++
+
+%% ── plumbing (byte-level; string-literal charlists avoided for portability) ──
+starts_with(Bin, Prefix) -> starts_with_(binary_to_list(Bin), Prefix).
+starts_with_(_, [])            -> true;
+starts_with_([C | T], [C | P]) -> starts_with_(T, P);
+starts_with_(_, _)             -> false.
+
+last_seg(Bin) -> last_seg_(binary_to_list(Bin), []).
+last_seg_([], Acc)      -> lists:reverse(Acc);
+last_seg_([47 | T], _)  -> last_seg_(T, []);      % reset at each '/'
+last_seg_([C | T], Acc) -> last_seg_(T, [C | Acc]).
+
+field(K, [{K, V} | _])  -> V;
+field(K, [_ | Rest])    -> field(K, Rest);
+field(_, [])            -> nil.