Schema Introspection api

[Ryang-21]

Export a typed introspection surface so external walkers don't depend on unchecked casts

The stellar-sdk JSON walker (SEP-0051 toJson/fromJson) currently
re-declares js-xdr's internal shapes as local interfaces and casts into them —
unverified by the compiler, so an internal rename would fail at runtime, not
compile time. This makes the walker surface real, versioned API.

• One *Schema interface per parameterized kind (StructSchema,
  UnionSchema, ArraySchema, FixedArraySchema, OptionSchema,
  LazySchema, OpaqueSchema, VarOpaqueSchema, StringSchema; kind: 'enum' added to EnumSchema), each extending XdrType<T> with kind
  narrowed to its literal. Factories return these types, so Color.entries
  and Tagged.cases typecheck with no casts. The exposure mirrors constructor
  inputs, so it adds essentially no new representation commitment.
• AnySchema — a closed union of all built-in schema shapes. Walkers cast
  once at the entry; switch (schema.kind) then genuinely narrows each branch
  (and gives exhaustiveness checking — the compiler catches a missing kind).
  Custom BaseType subclasses are documented as outside the closed union.
• opaque/varOpaque/string lengths made readonly public (walkers need
  them; previously private).
• The test suite includes a miniature schema-driven walker as a compile-time
  proof that the external walker can be written against public API alone.

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[Copilot]

Pull request overview

This PR introduces a typed schema-introspection API so external schema walkers (e.g., SEP-0051 JSON to/from) can rely on compiler-checked schema shapes instead of unchecked casts into js-xdr internals.

Changes:

• Adds *Schema introspection interfaces for compound schema kinds and updates factories to return those interfaces.
• Exports a closed AnySchema union intended for switch (schema.kind)-based walkers, plus exports of the new schema types from src/index.ts.
• Makes opaque/varOpaque/string length fields publicly readable and adds a new test demonstrating walker-style consumption.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/unit/introspection.test.ts	Adds a schema-driven “walker” test exercising the new introspection surface.
src/types/array.ts	Returns ArraySchema and exports the public array introspection interface.
src/types/enum.ts	Adds kind: 'enum' to EnumSchema for narrowing.
src/types/fixed-array.ts	Returns FixedArraySchema and exports fixed-array introspection interface.
src/types/lazy.ts	Returns LazySchema and exports lazy-schema introspection interface.
src/types/opaque.ts	Makes length publicly readable and returns OpaqueSchema.
src/types/option.ts	Returns OptionSchema and exports option introspection interface.
src/types/string.ts	Makes maxLength publicly readable and returns StringSchema.
src/types/struct.ts	Returns StructSchema and exports struct introspection interface.
src/types/union.ts	Returns UnionSchema and exports union introspection interface.
src/types/var-opaque.ts	Makes maxLength publicly readable and returns VarOpaqueSchema.
src/index.ts	Re-exports schema interfaces and defines PrimitiveSchema/AnySchema unions for external walkers.

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