Add opt-in schema-scoped PostgreSQL introspection

[Zetazzz]

Summary

• add a parameterized schema-scoped query to pg-introspection;
• retain the transitive relation, constraint, function-signature, type, range, index, inheritance, and extension dependencies needed by the selected schemas;
• let the existing PgIntrospectionPlugin opt in per PostgreSQL service;
• fail closed when the result crosses into an unapproved dependency schema or contains a missing type reference.

When scopedIntrospection is unset or false, the existing full-catalog query and default preset are unchanged. A regression test pins the stock SQL to its pre-refactor SHA-256.

Service configuration

Object.assign(makePgService({ schemas: ["app_public"], ... }), {
  scopedIntrospection: true,
  introspectionAllowedDependencySchemas: ["app_private", "extensions"],
  introspectionScopedCatalogTypes: "dependency-closure",
  introspectionCapabilityExtensions: ["pg_trgm"],
});

Scoped-only options without scopedIntrospection are rejected instead of being silently ignored. This PR does not add a replacement plugin, change the default preset, or alter PostgreSQL JIT/session settings.

Correctness coverage

The real PostgreSQL fixture covers cross-schema enum, domain, composite, range/multirange, function signature, foreign-key and inheritance dependencies; ordinary and pg_trgm GIN/GiST indexes; explicit extension capability metadata; unrelated schema exclusion; stock/scoped GraphQL SDL equality; runtime execution; and an unapproved-schema negative case.

Performance evidence

The same query/closure implementation was measured in constructive#1763 with 10 fresh-process samples per arm and identical JIT-off sessions. On the 65,037-pg_class fixture:

• schema build p50: 4,682 ms → 398 ms (-91.5%);
• introspection query p50: 3,116 ms → 111 ms (-96.5%);
• JSON payload: 163.9 MB → 1.06 MB (-99.4%);
• parsed classes: 64,220 → 270;
• stock/scoped schema hashes and runtime queries matched.

The small fixture showed a 1.9% build-time overhead, so this remains explicitly opt-in. The benchmark controlled JIT externally; this PR intentionally does not manage it.

Validation

• pg-introspection: 41 tests passed; build passed
• graphile-build-pg: 27 tests passed, including 5 real PostgreSQL scoped cases; build passed
• affected ESLint and Prettier checks passed
• changeset status and git diff --check passed