Add Binect SDK implementation, Explorer, and test suite

SDK (@binect/js):
- BinectClient with domain sub-clients (documents, sendings, accounts,
  attachments, invoices)
- HTTP Basic Auth, native fetch only (no runtime dependencies)
- TypeScript types matching Binect API vocabulary
- Status predicates and polling helpers in helpers.ts
- Structured error handling (BinectApiError, BinectAuthError)

Explorer:
- Standalone browser-based API explorer (explorer/index.html)
- Interactive testing without code

Tests:
- Unit tests for client, types, errors, helpers, http
- E2E tests for upload/delete and send/cancel workflows

Also includes:
- Architecture Decision Records (ADRs)
- Example DIN 5008 letter PDFs for testing
- API specification research notes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/types.test.ts

@@ -0,0 +1,52 @@
+import { describe, it, expect } from 'vitest';
+import {
+  DocumentStatus,
+  EnvelopeType,
+  FrankingType,
+  ProductionCountry,
+  ResponseFormat,
+} from '../src/types.js';
+
+describe('Enums', () => {
+  describe('DocumentStatus', () => {
+    it('has correct values', () => {
+      expect(DocumentStatus.IN_PREPARATION).toBe(1);
+      expect(DocumentStatus.SHIPPABLE).toBe(2);
+      expect(DocumentStatus.PRODUCTION_QUEUE).toBe(3);
+      expect(DocumentStatus.PRINTING).toBe(4);
+      expect(DocumentStatus.SENT).toBe(5);
+      expect(DocumentStatus.CANCELED).toBe(6);
+      expect(DocumentStatus.ERRONEOUS).toBe(7);
+    });
+  });
+
+  describe('EnvelopeType', () => {
+    it('has correct values', () => {
+      expect(EnvelopeType.DINLANG).toBe('DINLANG');
+      expect(EnvelopeType.C4).toBe('C4');
+    });
+  });
+
+  describe('FrankingType', () => {
+    it('has correct values', () => {
+      expect(FrankingType.UNSPECIFIED).toBe('UNSPECIFIED');
+      expect(FrankingType.STANDARD_FRANKING).toBe('STANDARD_FRANKING');
+      expect(FrankingType.DV_FRANKING).toBe('DV_FRANKING');
+    });
+  });
+
+  describe('ProductionCountry', () => {
+    it('has correct values', () => {
+      expect(ProductionCountry.UNSPECIFIED).toBe('UNSPECIFIED');
+      expect(ProductionCountry.DE).toBe('DE');
+      expect(ProductionCountry.AT).toBe('AT');
+    });
+  });
+
+  describe('ResponseFormat', () => {
+    it('has correct values', () => {
+      expect(ResponseFormat.FULL).toBe('FULL');
+      expect(ResponseFormat.SHORT).toBe('SHORT');
+    });
+  });
+});