Summary

concurrency: true on a command declared a guarantee that has never been in force.

Fixed

A command scoped with concurrency: true now actually participates in the concurrency check. The scope carries an expected sequence number resolved by the configured concurrency strategy, instead of a hardcoded EventSequenceNumber.Unavailable that the kernel skipped.
Declaring concurrency: true no longer leaves a command with weaker protection than declaring nothing. The scope it built was not NotSet, so it displaced the strategy the event sequence would otherwise have applied.
The scope is bound to the event source being appended to. A null event source id made the kernel compute the tail across every event source sharing the type.
A command appending across streams gets a concurrency scope per target event source, instead of one scope carrying a single stream's expected tail applied to all of them.