Release v19.106.6

Groundwork for per-store projection generations: each event store can now carry a durable compatibility floor that the kernel checks before it changes anything at startup. Nothing in this release writes a floor, so behavior is unchanged.

Added

• A per-event-store generation floor that records the minimum generation protocol, the capabilities a kernel must support and the store incarnation, stored in MongoDB, SQL and in-memory storage with conditional writes that never lower it. (part of #5223) (part of #5169)
• Startup admission: before registering event types, applying patches or recovering jobs, the kernel checks the floor of the System store and every event store and refuses to start when it does not meet it. Stores without a floor are only read. (part of #5169)

Changed

• A kernel state reset keeps every event store's generation floor. (part of #5223)