Function createSerialSession

Create a v2 SerialSession.

This release wires the internal read pump (#202) and the internal send queue (#203) into the session so that connect$, disconnect$, receive$, and send$ all operate end-to-end. Error handling is centralised through a single reportError helper (#204) so every failure path normalises through normalizeSerialError and emits on the one errors$ channel.

Key behaviors:

• isBrowserSupported() returns whether navigator.serial is available.
• state$ replays the current lifecycle state driven by SessionStateMachine.
• connect$() opens a user-selected port, starts the internal read pump, and transitions idle -> connecting -> connected.
• disconnect$() stops the read pump, closes the port, and transitions connected -> disconnecting -> idle.
• receive$ emits UTF-8 decoded text chunks pushed by the pump. It is not subscription-lazy - the pump is started by connect$ and decoded text is multicast to all subscribers; late subscribers see only new data.
• lines$ emits the same decoded stream split into line-terminated segments (\n, \r\n); a trailing line without a terminator is buffered. It is also not subscription-lazy relative to the pump.
• send$ enqueues each payload on an internal FIFO queue so concurrent subscribers are written to the port in call order. String payloads are UTF-8 encoded through a shared TextEncoder.
• errors$ multiplexes every SerialError produced by the session. Connect / read / close failures are treated as fatal and also drive state$ to 'error'; write failures are non-fatal and do not mutate state$ because a real connection loss will be observed by the read pump on the next tick anyway.