Release v7.54.0

Guidance on keeping the event log free of non-facts: what is and is not an event, how observation commands avoid repeats, and how to roll out a cleanup that removes or reshapes many event types.

Added

• Event modeling skill: a section and a reference on event log hygiene (no ticks, signal events, "last looked" timestamps, telemetry or non-answers; orchestrate with jobs, state machines, reminders, command operations or ICommandPipeline), zero repeats, …Changed naming, and measuring the log for identical consecutive events.
• Event modeling skill: a reference for rolling out a release that removes or reshapes many event types (scale to zero, backup, dry run with fuses, wipe the whole schema registry, BSON NumberLong, replay verification).
• Glossary entries for observation command and signal event.

Changed

• Vertical slices rule: observation commands append only changes, compared against a [Passive] model or the stream's own events, with two specifications each; counters built with [Increment]/[Decrement] need commands that refuse non-transitions.
• Reactors rule and skill: a reactor added in a release catches up the whole history on first registration and [OnceOnly] does not cover that; sweeps must not decide from read models that are replaying.
• Projection skill points to the counter transition requirement.