Make AimX remote access spawn free

[lxsaah]

Make AimX remote-access and WS client connector spawn-free

Closes #114. Implements design 030; finishes the "Group 4" work design 028 deferred.

Why

After #88 every runtime.spawn(...) was gone — except nine bare tokio::spawn calls in AimX (3) and the WS client connector (6). They didn't depend on the Spawn trait (gated via #[cfg(feature = "std")]), so the trait could ship, but the dynamic-fan-out shape needed a nested FuturesUnordered rewrite that was out of scope for #88. This PR does that rewrite. Zero tokio::spawn remain in aimdb-core or the connectors; cancellation collapses to dropping the future.

What changed

AimX (aimdb-core/src/remote/)

• supervisor.rs — accept loop owns a FuturesUnordered<BoxFuture> of per-connection handlers (select! { biased; }). max_connections (previously declared-but-unread) is now actually enforced: over-cap connections are refused pre-handshake.
• handler.rs — each record.subscribe pushes a future onto a per-connection FuturesUnordered. The two-task chain per subscription collapses into one run_subscription future.
• stream.rs (new) — stream_record_updates adapts a JsonBufferReader into a Stream<Item = serde_json::Value> via unfold. Drop to cancel.
• AimDb::subscribe_record_updates deleted — only caller was the AimX handler.

Cancellation: Notify, not AtomicBool
record.unsubscribe fires an Arc<Notify> raced against stream.next() in a biased select!. Cancellation is immediate even on a quiet record — the previous atomic-flag design could pin a buffer reader indefinitely until the next event.

WS client connector
All six tokio::spawn sites collapse into one infrastructure future owning a FuturesUnordered. The reconnect watcher sends NewLoops { write_sink, read_stream, write_rx } over an mpsc to the outer future instead of spawning. WsClientConnectorImpl::connect() now returns Result<(Self, BoxFuture), String>; internal-only.

Breaking changes

• AimxConfig::subscription_queue_size removed → replace with .max_subs_per_connection(n) (default 32) or delete the call.
• AimxConfig::max_connections (default 16) is now actually enforced.
• AimX Welcome.max_subscriptions now reports max_subs_per_connection (32) instead of the dead subscription_queue_size (100).
• record.subscribe response no longer carries queue_size; result is { "subscription_id": "..." }.

Tests

New #[cfg(all(test, feature = "std"))] tests in handler.rs / stream.rs:

• dropping_subs_set_drops_subscription_stream — drops the FuturesUnordered, verifies the underlying stream is dropped.
• dropping_subs_set_drops_inner_stream_state — stronger version with a real channel-backed stream.
• unsubscribe_terminates_subscription_immediately — notify_one() exits the future parked on stream.next() with no further values needed.
• unfold_skips_lag_and_terminates_on_closed — covers BufferLagged skip and BufferClosed exit.

make check / make all green.

Docs

• New: 030-M13-aimx-remote-spawn-free.md.
• Updated: 028-M13-remove-spawn-trait.md bridge-state retro-edited to point at 030; both changelogs.

Out of scope

Lifting #[cfg(feature = "std")] on AimX — shape is now runtime-agnostic, but transport (UDS, tokio I/O) is still std-only.

[Copilot]

Pull request overview

This PR completes the “spawn-free” refactor for AimX remote access and the WebSocket client connector by removing remaining tokio::spawn fan-out points and driving per-connection / per-subscription (and per-reconnect) loops via nested FuturesUnordered owned by a single infrastructure future, with cancellation via dropping futures.

Changes:

• Refactors AimX supervisor + per-connection handler to own nested FuturesUnordered sets (no per-connection / per-sub tokio::spawn), and enforces max_connections / max_subs_per_connection.
• Introduces stream_record_updates helper returning a Stream for record updates, and removes the old AimDb::subscribe_record_updates spawn-based helper.
• Refactors WS client connector to return an infrastructure future that owns its background loops and uses an mpsc (NewLoops) to swap read/write loops on reconnect (no spawns).

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
examples/remote-access-demo/src/server.rs	Updates demo config API usage to max_subs_per_connection.
examples/remote-access-demo/src/client.rs	Updates demo client to new record.subscribe request/response shape (no queue_size).
docs/design/030-M13-aimx-remote-spawn-free.md	New design doc describing the spawn-free implementation and rationale.
docs/design/028-M13-remove-spawn-trait.md	Updates “Group 4” bridge-state discussion to point to design 030 as resolved.
aimdb-websocket-connector/src/client/connector.rs	Reworks WS client connector to spawn-free nested FuturesUnordered model + reconnect loop handoff via NewLoops.
aimdb-websocket-connector/src/client/builder.rs	Prepends the new connector infrastructure future to the returned futures list.
aimdb-websocket-connector/CHANGELOG.md	Documents the WS client spawn-free refactor and connect signature change.
aimdb-core/src/remote/supervisor.rs	Replaces per-connection tokio::spawn with a supervisor-owned FuturesUnordered and enforces max_connections.
aimdb-core/src/remote/stream.rs	Adds stream_record_updates (unfold-based) stream adapter + tests.
aimdb-core/src/remote/mod.rs	Updates module docs and exports stream module under std.
aimdb-core/src/remote/handler.rs	Replaces per-subscription spawning with per-connection FuturesUnordered of subscription futures; unsubscribe uses Notify. Adds tests.
aimdb-core/src/remote/config.rs	Removes subscription_queue_size, adds max_subs_per_connection, updates defaults/tests.
aimdb-core/src/builder.rs	Deletes spawn-based AimDb::subscribe_record_updates helper.
aimdb-core/CHANGELOG.md	Documents AimX spawn-free refactor + breaking config/protocol shape changes.

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