[Rust] Extract shared mux core

[danilonajkov-db]

What changes are proposed in this pull request?

For easier and cleaner multiplexing for arrow streams I extract the existing multiplexed-stream routing and lifecycle logic into a private MuxCore. I also add a generic MuxLane with the main methods needed for multiplexing. Then we maintain very small implementations for grpc and arrow streams.

MultiplexedStream            public API (thin wrapper, gRPC-specific bits)
   │  core: MuxCore<ZerobusStream>
   ▼
MuxCore<S: MuxLane>          policy: spreading work over N lanes, poisoning, lifecycle
   │  streams: Vec<S>
   ▼
MuxLane (trait)              mechanism: what a single transport stream must provide
   └─ impl for ZerobusStream (gRPC); Arrow lane comes in #866

Stack base: main, including merged Avro PR #857. Follow-ups: #866 (Arrow API), then #867 (lifecycle tests).

How is this tested?

• Existing gRPC mux integration suite: 24 passed.
• SDK Clippy with all features and all targets, warnings denied.
• Full Rust make test and make lint matrices passed on the combined implementation.
• Rust formatting and diff whitespace checks.