fix: never fail the start page over a query answer that came back short

[ashleycaselli]

Closes #623.

The home page reported Configured home resource ... could not be found whenever the resource repository had latched a snapshot and that snapshot had no entry for the configured id. Latching, though, took any answer at face value — an answer with no rows at all included.

That is the wrong reading of an empty answer. The query API is a set of instances with independently built indexes, so a short or empty result set is a routine kind of failure and not news that the resources are gone. One such answer latched an empty snapshot, ran removeStale() over it (wiping every MaintainedResource instance), and turned a passing API hiccup into the page declaring its own configuration broken — with the persisted cache that could have covered it overwritten along the way.

What now stands between such an answer and the page

An emptied answer is not latched	build() keeps the current snapshot when a new answer carries no resources at all and the current one has some, so what was fetched before — or restored from the persistent cache after a restart — goes on being served, instances and all.
A resource is not forgotten	Every resource a snapshot ever held is kept by id, and findLastKnownById() falls back to it. findById() keeps its strict answer for the many callers that use it to ask whether an IRI is a maintained resource.
"Not found" has to be earned	isAbsent() replaces isReady() as what the home page asks: a snapshot is latched, it holds resources, and neither it nor any earlier answer carried this id.

So the notice is reserved for a genuine misconfiguration — the configured IRI is none of the resources this instance knows, and never was. Anything else keeps the last known resource on screen, or goes on waiting for one.

The trade-off

An instance whose query API reports no maintained resources at all now waits on the loading state instead of showing the notice, because that case cannot be told apart from an answer that failed to carry them. That is the trade the issue asks for: the start page should not fail.

Tests

Four new, driving the repository through the answers it has to survive — an answer with nothing in it, an answer short of one resource, an id no answer ever carried, and a repository that knows nothing yet. All stubbed, so none of them goes near the network.

Suite: 1263 tests, green.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TtcVArNfYKiiTVowsdkF44