Replaces the per-tag Plc.ReadAsync loop in S7Driver.ReadAsync with a
batched ReadMultipleVarsAsync path. Scalar fixed-width tags (Bool, Byte,
Int16/UInt16, Int32/UInt32, Float32, Float64) are bin-packed into ≤18-item
batches at the default 240-byte PDU using S7.Net.Types.DataItem; arrays,
strings, dates, 64-bit ints, and UDT-shaped types stay on the legacy
ReadOneAsync path. On batch-level failure each tag in the batch falls
back to ReadOneAsync so good tags still produce values and the offender
gets its per-item StatusCode (BadDeviceFailure / BadCommunicationError).
100 scalar reads now coalesce into ≤6 PDU round-trips instead of 100.
Closes#292
