D333 — An empty usage store says since when, and stays local state

Decision. With no usage report, kb_status.usage carries no_report_since, the server process's start time, beside no_data. docs/deployment.md names usage.json among the .cartographer/ files a restart on an ephemeral data dir loses. The store itself stays where D326 put it.

Why. A container that clones its KBs at start onto an emptyDir empties the store at every restart: kb_status then looked exactly like a KB no client had ever reported to, right after an upgrade rollout. The start time lets a reader tell "nothing in the last minute" from "nothing for weeks" without a second store.

Alternatives rejected. - A configurable state dir separate from the KB clone: a new config key and path rule for a signal D326 already calls disposable; an operator who wants it to survive mounts .cartographer/ on the persistent volume, which also keeps the conflict registry. - Persist a "reports seen" marker elsewhere: it would live on the same ephemeral disk, or in git, where D326 decided usage does not belong.

Consequences. no_report_since is a fact about the process, not about the KB: on a persistent store with no reports it is still true, just not the cause.