This PR implements two XDR (External Data Representation Standard) decoder classes to complement the existing XDR encoder functionality, providing complete RFC 4506 compliant binary data processing capabilities.

Implementation

XdrDecoder - Lower-level decoder

A foundational decoder class that uses the Reader interface for explicit type-based XDR decoding:

import { XdrDecoder } from '@jsonjoy.com/json-pack/xdr';

const decoder = new XdrDecoder();
const data = new Uint8Array([0, 0, 0, 42, 0, 0, 0, 5, 104, 101, 108, 108, 111, 0, 0, 0]);

decoder.reader.reset(data);
const value = decoder.readInt();        // 42
const length = decoder.readInt();       // 5
const text = decoder.readString();      // "hello"

XdrSchemaDecoder - Higher-level schema-aware decoder

A schema-driven decoder that automatically handles complex XDR structures based on type definitions:

import { XdrSchemaDecoder, XdrUnion } from '@jsonjoy.com/json-pack/xdr';

const schema = {
  type: 'struct',
  fields: [
    [{ type: 'int' }, 'id'],
    [{ type: 'string' }, 'name'],
    [{ 
      type: 'union',
      arms: [[0, { type: 'string' }], [1, { type: 'int' }]]
    }, 'contact']
  ]
};

const decoder = new XdrSchemaDecoder();
const result = decoder.decode(binaryData, schema);
// Returns: { id: 123, name: "Alice", contact: XdrUnion { discriminant: 0, value: "alice@example.com" } }

Key Features

• RFC 4506 Compliance: Proper 4-byte alignment, big-endian byte order, and padding handling
• Complete Type Support: All XDR primitives (int, unsigned_int, hyper, float, double, boolean, etc.)
• Schema Integration: Full compatibility with existing XDR schema types from types.ts
• Complex Structures: Support for structs, unions, arrays, enums, and nested compositions
• Round-trip Compatibility: Seamless integration with existing XdrEncoder and XdrSchemaEncoder
• Size Validation: Automatic constraint checking for variable-length data
• Error Handling: Comprehensive validation and meaningful error messages

Testing

Added 65 comprehensive tests covering:

• All primitive type decoding with round-trip verification
• Complex nested structures and edge cases
• Size constraint validation and error conditions
• Integration with existing encoder classes
• Special values (NaN, Infinity, boundary conditions)

All existing tests continue to pass, ensuring backward compatibility.

Fixes #55.

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