server: fix client switching desync on reconnect

[justdoGIT]

Fixes #65

Using a Slab for client storage causes client indices to become non-contiguous when clients disconnect and reconnect. Because the switching logic assumed contiguous 0..clients.len() indexing:

let exists = |idx| idx == 0 || clients.contains(idx - 1);
loop {
    current = (current + 1) % (clients.len() + 1);
    if exists(current) {
        break;
    }
}

A reconnected client assigned a slab key > 0 while earlier slots are vacant would fail exists(current) and immediately cycle back to current = 0 (server), producing the exact symptom described in #65:

INFO rkvm_server::server: Switched client idx=0

The switch hotkey becomes completely unresponsive to that client until the server process is restarted.

This PR replaces Slab with a Vec for client storage so indices remain contiguous, eliminating index drift and ensuring reliable round-robin switching across disconnects and multiple clients.

[Copilot]

🟡 Changes recommended

The new Vec compaction/removal paths don’t consistently adjust both current and previous, which can lead to incorrect targeting and potential out-of-bounds panics.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR fixes client switching becoming unresponsive after disconnect/reconnect by changing client storage from Slab (non-contiguous keys after removals) to Vec (contiguous indices), so the round-robin switch (current + 1) % (clients.len() + 1) remains valid across reconnects.

Changes:

• Replaced client storage from Slab<(Sender<_>, SocketAddr)> to Vec<(Sender<_>, SocketAddr)>.
• Simplified the switch-hotkey logic to a single modulo step (no “exists” scan).
• Adjusted client removal handling when a send fails to keep current consistent with shifted Vec indices.

File summaries

File	Description
rkvm-server/src/server.rs	Switch client bookkeeping from Slab to Vec to keep indices contiguous and make switching reliable across disconnects/reconnects.

Review details

Suppressed comments (1)

rkvm-server/src/server.rs:259

• When a send fails and you remove(idx - 1), the vector shifts. The code updates current but not previous, even though previous can later be used as an index (idx = previous while changed is true). This can leave previous out of range and cause an out-of-bounds panic on clients[idx - 1], especially if the removed client was previous or was before it.

                        if clients[idx - 1].0.send(Update::Event { id, event }).await.is_err() {
                            clients.remove(idx - 1);

                            if current == idx {
                                current = 0;
                            } else if current > idx {
                                current -= 1;
                            }
                            break;

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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