sources::window resolves an event id to its chunk, then walks chunks of the
same room both ways (5 per read, at most 6 reads a side), dedupes by event id
and cuts to before/after with more_before/more_after. An id the index lacks is
a typed SelectionInvalid on event_id naming the three reasons; an id in two
chunks is an index fault. No embedder, no Synapse. ChatFilter gains event_id
and chunk-start bounds, chat_scroll gains an order, the wire gains ChatWindow
request/response, the CLI gains `huginn chat window`.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
