refactor: make the filter-polarity trap inexpressible

service.SpaceFilter meant "empty membership selects nothing" — a newly
created project has no members — while search.Query.Spaces was a bare
[]core.SpaceRef whose empty case meant every space. Passing a project's
members into a query therefore turned an empty project into the whole
corpus: a silent scope inversion, invisible when it happens, and passing
every test written with a non-empty project.

The filter moves to core.SpaceFilter with unexported fields, and
search.Query takes it whole. There is no slice to hand over any more, so
the inversion cannot be written. Its three states are distinct: every
space (EverythingFilter), exactly these — possibly none — (SpacesFilter),
and the zero value, which is neither. Search refuses the zero value
rather than defaulting it, because both plausible defaults are wrong for
one of the two callers that can produce one, and returns no hits for a
filter that selects no space without asking the index.

service.SpaceFilter is now an alias for the core type, so ResolveProject
and its callers keep their names. Tests that built a Query without a
scope now say core.EverythingFilter(), which is what they always meant.

feat: projects — a saved filter over one global index, not a container

A project is a named space set; querying one filters the single global
index. The meta-project is an implicit filter at a reserved address rather
than a row: a stored +everything would need a sync job on every space
creation, and its one failure mode is silently omitting a space.

SpaceFilter distinguishes All from an empty member list, because a freshly
created project has no members and must mean selects-nothing. Collapsing
the two would make every new project silently match the whole corpus.