Enhance migration and changelog documentation

CHANGELOG.md

@@ -25,6 +25,8 @@ A complete rewrite of the library. The runtime `xdr.config(...)` schema-definiti
 * **`lazy(() => schema)`** builder for defining recursive and forward-referencing schemas.
 * **`BaseType` / `XdrType<T>`** are exported as the base class and interface for all schemas, along with a `DecodeOptions` `{ maxDepth }` option preserved from the prior recursion-depth guard.
 * **`enumType` reserved-name and duplicate-value validation** — throws if a member name collides with a schema property (`name`, `kind`, `encode`, …) or if two members share a wire value.
+* **Schema introspection API.** Every builder now returns a typed schema interface (`StructSchema`, `UnionSchema`, `ArraySchema`, `FixedArraySchema`, `OptionSchema`, `LazySchema`, `OpaqueSchema`, `VarOpaqueSchema`, `StringSchema`, `EnumSchema`) whose `kind` property is narrowed to a literal, plus a closed `AnySchema` union (with `PrimitiveSchema` covering the argument-less kinds). Schema-driven walkers — such as generic JSON converters — can cast once to `AnySchema` and `switch (schema.kind)` with exhaustiveness checking, instead of re-declaring internal shapes and casting. Supporting surface: `enumType` schemas expose `nameByValue` (wire value → member name); `opaque`/`varOpaque`/`string` expose their `length`/`maxLength`; `EnumMember`, `EnumName`, `Field`, `UnionArm`, and `UnionCase` helper types are exported.
+* **`encode(value, { maxDepth })`** — the depth guard also applies to encoding (via the exported `EncodeOptions`), failing with `XdrError` on cyclic values fed to recursive schemas instead of overflowing the call stack.
 
 ### Changed
 * **Build chain modernized:** Webpack + Babel → Rollup + esbuild; output is a clean dual ESM/CJS bundle with `.d.ts` emission via `rollup-plugin-dts`.

MIGRATION.md

@@ -310,12 +310,13 @@ Result.validate({ type: ResultType.error, code: 7 }); // true
 Result.validate({ type: ResultType.error }); // false
 ```
 
-The recursion-depth guard is now a decode option rather than a constructor
-argument:
+The recursion-depth guard is now an encode/decode option rather than a
+constructor argument:
 
 ```ts
 Result.decode(bytes, { maxDepth: 100 });
 Result.validateXdr(bytes, { maxDepth: 100 });
+Result.encode(value, { maxDepth: 100 }); // guards against cyclic values
 ```
 
 ## Errors
@@ -395,6 +396,33 @@ data, so build a new object instead of mutating a decoded value.
 through composition, so prefer deriving types from schemas instead of
 duplicating object shapes by hand.
 
+### Schema introspection
+
+Each builder returns a typed schema interface (`StructSchema`, `UnionSchema`,
+`ArraySchema`, …) with `kind` narrowed to a literal. To write a generic
+schema-driven walker (e.g. a JSON converter), cast the root once to the closed
+`AnySchema` union and switch on `kind` — each branch narrows to the matching
+interface, with compiler-checked exhaustiveness:
+
+```ts
+import type { AnySchema } from '@stellar/js-xdr';
+
+function describe(schema: AnySchema): string {
+  switch (schema.kind) {
+    case 'struct':
+      return `struct { ${schema.entries.map(([name]) => name).join(', ')} }`;
+    case 'enum':
+      return `enum ${schema.name}`;
+    case 'array':
+      return `${describe(schema.element as AnySchema)}<>`;
+    // ...remaining kinds
+  }
+}
+```
+
+`AnySchema` covers only the schemas this package creates; custom `BaseType`
+subclasses must be handled before the cast.
+
 For very large XDR definitions, or definitions with cyclic references, deriving
 all value types through `Infer` can put significant pressure on the TypeScript
 compiler and language server. In those cases, prefer explicit hand-written value