refactor(irc): select directly over connection receivers

[olivermrose]

Replaces the per-connection forwarding task that repackaged ConnectionIncomingMessage into ClientLoopCommand::IncomingMessage with a StreamMap of UnboundedReceiverStream keyed by connection id, multiplexed into the client loop's main select!.

This removes the Weak<Sender> indirection used to break the forward-task <-> client-channel cycle, the tx_kill_incoming oneshot on PoolConnection, and the IncomingMessage command variant.

Assisted by Claude.

[Copilot]

Pull request overview

Refactors the IRC client event loop to multiplex per-connection incoming message receivers directly in the main select!, removing the prior per-connection forwarding task and associated indirections.

Changes:

• Replace the ClientLoopCommand::IncomingMessage forwarding path with a StreamMap<usize, UnboundedReceiverStream<_>> multiplexed in the client loop.
• Remove PoolConnection’s tx_kill_incoming oneshot and Drop impl used to terminate the forwarding task.
• Add tokio-stream dependency to support StreamMap and UnboundedReceiverStream.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src-tauri/src/irc/client/pool_connection.rs	Removes forwarding-task shutdown plumbing (tx_kill_incoming + Drop) from PoolConnection.
src-tauri/src/irc/client/mod.rs	Updates ClientLoopWorker::spawn callsite to new signature (no Weak sender).
src-tauri/src/irc/client/event_loop.rs	Introduces StreamMap-based multiplexing of connection incoming streams; removes IncomingMessage command variant and forward-task logic.
src-tauri/Cargo.toml	Adds tokio-stream dependency (with sync feature) for stream wrappers/multiplexing.
src-tauri/Cargo.lock	Locks tokio-stream and transitive dependencies.

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