fix(graphile-schema): correlate SDL and _meta metadata to one schema build

[pyramation]

Summary

Fixes constructive-io/constructive-planning#1331 — offline schema builds could return one schema's GraphQL artifacts paired with another schema's _meta metadata, because callers read the process-global _cachedTablesMeta as a separate operation after an awaited build (A write → B write → A read).

The result now comes from one correlated build boundary keyed on the exact final executable GraphQLSchema:

// graphile-meta: schema-keyed accessor over the existing WeakMap
export function getTablesMetaForSchema(schema: GraphQLSchema): TableMeta[] | undefined

// graphile-schema
export async function buildSchemaArtifacts(opts): Promise<{ sdl: string; tablesMeta: TableMeta[] }> {
  const { schema } = await makeSchema(preset);
  // execute shallow _meta against this schema, then:
  tablesMeta = getTablesMetaForSchema(schema) ?? [];   // same GraphQLSchema identity
  return { sdl: printSchema(lexicographicSortSchema(schema)), tablesMeta };
}

// existing APIs become projections
buildSchemaSDL(opts)        // => artifacts.sdl
buildIntrospectionJSON(opts) // => artifacts.tablesMeta
DatabaseSchemaSource.fetch() // consumes { sdl, tablesMeta } from the same call

• _cachedTablesMeta and its plumbing (cache.ts, the finalize mirror hook, all re-exports) are deleted outright — no deprecated compatibility export. Live _meta behavior is unchanged and stays WeakMap-scoped.
• If the meta plugin is disabled, tablesMeta is an explicit [] (never stale metadata from an unrelated build).
• An internal test-only _onMetaCollected hook lets regression tests deterministically force the unsafe A write → B write → A read schedule (ordinary Promise.all stress was shown to be too weak/flaky in the issue).

Tests

• graphile/graphile-schema/__tests__/build-schema-artifacts.test.ts (live PG): sequential, forced-interleave, uncoordinated-concurrent, and plugin-disabled cases — each caller gets only its own metadata.
• graphql/codegen/src/__tests__/introspect/database-source.test.ts (unit): DatabaseSchemaSource.fetch() returns introspection and tablesMeta belonging to the same schema even when another build completes mid-fetch.
• CI: graphile/graphile-meta and graphile/graphile-schema are now run in the pg-graphql batch (previously only built).

Link to Devin session: https://app.devin.ai/sessions/53231fe0747e4f0787814196eea57ee4
Requested by: @pyramation

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