Add a completion acceptance callback

[hiroki23]

Summary

• add an optional OnAccept callback to each completion candidate
• invoke it only after the candidate is inserted into the real input line
• cover callback timing and cursor state

This lets callers associate application-specific follow-up behavior with the exact menu candidate the user accepted, without affecting virtual candidate selection.

Tests

• GOCACHE=/tmp/type-rb-go-cache go test ./...