Support @defer and @stream in the e2e test suite

[captbaritone]

Adds @defer/@stream support to the markdown-driven e2e suite, plus three fixtures covering it.

Incremental delivery did not work in these fixtures, and neither reason it did not work was loud.

The compiler stripped @defer from the query text

relay-compiler does not define @defer/@stream — it reads them off the schema, and it has to be the server schema. Grats has no reason to emit them (they are executor-level, not part of anyone's resolvers), so runFixture now appends the definitions to the generated schema.graphql between the Grats and relay-compiler steps.

Supplying them through schemaExtensions instead does compile, which is the trap: skip_client_directives then strips the directive out of the printed query text as a client-only directive, so the server is never asked to defer. The field comes back inline while the generated artifact still holds a Defer node waiting for a patch that never arrives, so the field reads as missing. The fixture passes with its deferred content silently empty.

graphql-js ignored the directive regardless

collectFields checks schema._enableDeferStream and returns before it ever looks at @defer. The flag only takes effect through the GraphQLSchema constructor and does not round-trip through toConfig(), so GratsNetwork re-wraps the Grats schema to turn it on.

Trailing payload

GratsNetwork also drops graphql-js's closing {hasNext: false} payload, which carries neither data nor errors and so is not a GraphQLResponse. Relay reported it as No data returned for operation and logged a warning. Nothing is lost: Relay learns the request finished when the observable completes. @defer rarely hits this — graphql-js folds hasNext: false into the final patch when it knows that patch is last — but @stream emits the trailing payload every time.

Fixtures

Fixture	Covers
defer/basic	Deferred fragment renders once its patch is normalized
defer/suspense-fallback	Fallback is on screen while the patch is outstanding
stream/list-field	Streamed list items; the trailing-payload path

Fixtures need no per-fixture setup now — spread with @defer and give the field an async resolver.

defer/suspense-fallback uses a technique worth flagging in review: an async resolver settles a microtask later, so asserting on the pending state would be a race. Instead server.ts exports a plain non-schema releaseBio() that a button calls, since fixture server and client share a module realm. That makes wait "Loading bio..." deterministic. Documented in the writing guide.

@stream is included because the graphql-js flag enables both; declaring only @defer would leave half of it enabled and untested. The stream fixture is what caught the trailing-payload bug.

Test plan

yarn test:e2e — the three new fixtures pass, and the rest of the suite is unchanged by the harness edits (verified by stashing them and re-running).

Four fixtures (errors/nested-catch-field-error{,-flag-enabled}, pagination/root-catch-{load-next,refetch}) fail in my working tree, but they fail identically without these changes — unrelated in-progress local work, not caused by this PR and not touched by it.