feat(graphile-schema): add buildIntrospectionJSON() for database metadata export

[pyramation]

Summary

Adds buildIntrospectionJSON() to graphile-schema — a thin wrapper around buildSchemaSDL() that returns the TableMeta[] array populated by MetaSchemaPlugin during the schema build. This enables downstream consumers (e.g. constructive-db's generate:schemas pipeline) to export structured introspection JSON alongside .graphql SDL files, which can then drive blueprint type codegen instead of hand-coding structural types.

Changes:

• New file graphile-schema/src/build-introspection.ts — buildIntrospectionJSON(opts) calls buildSchemaSDL, then returns a shallow copy of _cachedTablesMeta
• graphile-settings/src/plugins/index.ts — re-exports all 14 meta-schema types (TableMeta, FieldMeta, TypeMeta, etc.)
• graphile-schema/src/index.ts — re-exports buildIntrospectionJSON + all meta-schema types from graphile-settings

Review & Testing Checklist for Human

• Side-effect reliance: buildIntrospectionJSON depends on buildSchemaSDL populating the module-level _cachedTablesMeta as a side-effect. Verify this is acceptable for your use case (concurrent calls would clobber the shared cache). Only a shallow copy is returned — mutations to nested objects would affect the cache.
• Type completeness: Confirm all types from meta-schema/types.ts that downstream consumers need are included in the 14 re-exported types. If the types file has additional exports (sub-types of QueryMeta, etc.), they may need to be added later.
• No tests included: This PR has no unit tests for buildIntrospectionJSON. Consider whether a test (even a smoke test verifying the function returns a non-empty array against a test database) should be added before or after merge.

Notes

• This is the first half of a two-repo effort. The follow-up PR on constructive-db will call buildIntrospectionJSON() in generate-schemas.ts and write the result to introspection.json, which generate-types.ts will then consume to derive blueprint structural types.
• BuildIntrospectionOptions is a type alias for the existing BuildSchemaOptions — introduced for API clarity so callers don't need to know the SDL function is involved.

Link to Devin session: https://app.devin.ai/sessions/ce1b2dafd42341bea5d7793b0c05ba6c
Requested by: @pyramation

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