graph: pin the introspection schema to what the executor can serve

The introspection schema comes from gqlparser's built-in prelude; the
code that executes it is generated by gqlgen. Nothing tied the two
together, and gqlgen's generated executor answers a field it does not
know with panic("unknown field"), recovered into "internal system
error". A gqlparser that has learned a newer edition of the
introspection schema than the pinned gqlgen can execute therefore leaves
/query advertising fields that fail when a consumer selects them —
which matters here, because this endpoint is meant to be federated.

The new test derives the field list from whichever prelude is in use
rather than hard-coding one, and checks every declared field of the six
introspection types is selectable through a path this schema really has.
Against gqlparser v2.5.36 it fails on __Type and __InputValue, the two
types v2.5.22 added fields to; at the pinned v2.5.21 it passes.

Also corrects generate.go. Its note explained why the directive resolves
gqlgen through the module graph, which is true, but read as though that
made codegen work. It does not: v0.17.36 re-emits an existing resolver's
doc comment without its // markers and leaves schema.resolvers.go as
invalid Go. v0.17.94 fixes that and is blocked on sr-ht-core, so the
note now says so and says where the bump belongs.