Caches the latest scenario extraction report and tells subscribers when it changes.

Why this owns its own subscriber set

It used to call Phoenix.PubSub.broadcast/3 against Foundry.PubSub, which put phoenix_pubsub in the library's dependency list and, worse, put a supervision tree there: the pubsub process had to already be running before a mix task could ask this module anything. ADR-035 moved everything with a supervision tree to the Studio, so the cache monitors its own subscribers instead. One process, no dependency, and the message a subscriber receives is unchanged -- {:scenarios_updated, report}.

get/0 answers nil rather than raising

ADR-036 runs the library inside a target project as a mix task, where nothing starts this GenServer. Foundry.Context.ProjectContext already treats nil as "no cache, extract now", so the absent process is a cache miss and not a failure. A GenServer.call to an unregistered name exits, which would have killed the mix task with a message about :noproc rather than about scenarios.

Boot does not extract

⚠️ It used to send(self(), :extract_static) here, and that extraction ran against File.cwd!() -- the Studio's directory, never the target's. Every subscriber was then handed a report describing Foundry's own repository, which is how the coverage panel came to say "105 nodes still have no scenario coverage" (Foundry's node count) about a project with 344 nodes. Coverage for a target is computed in that target's subprocess under ADR-036; this cache now only holds what mix foundry.scenarios.extract explicitly hands it.