switch printer FSM to looplab/fsm

[rusq]

Summary

Refactors the LX-D02 print-job state machine to use github.com/looplab/fsm and adds job-scoped routing for FSM side effects. Internal print events now dispatch directly to the owning print job instead of sharing the notification queue, while printer notifications remain best-effort and non-blocking.

Why

The previous switch-based FSM mixed internal packet-stream completion with printer 5a06 completion and used shared channels that could deadlock or receive stale events. Follow-up review also found that required internal events could be dropped when the notification queue was full, and stale goroutines from a canceled print could affect the next print.

Changes

• Add a per-print printJob owner for the FSM, notification queue, completion channel, cancellation, and packet-stream cancellation.
• Split internal eventPacketsSent from printer eventNotificationFinished.
• Dispatch required internal events directly through the owning job FSM so they cannot be dropped by a full notification queue.
• Ignore stale job events after cancellation or replacement by a later print.
• Return print failure/cancellation errors through single-shot completion handling.
• Keep public printing APIs unchanged.
• Update FSM tests to cover success, failures, cancellation, hold/retransmit, duplicate completion, full notification queues, stale init completion, stale packet stream events, and worker behavior with no active print.

Validation

• go test ./...
• go test -race ./...