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>

CLAUDE.md

@@ -2,38 +2,57 @@
 
 This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
 
+## Build Commands
+
+```bash
+npm install          # Install dependencies
+npm run build        # Build TypeScript to dist/
+npm test             # Run tests with vitest
+npm run typecheck    # Type check without emitting
+```
+
 ## Project Overview
 
 Binect-JS is a JavaScript/TypeScript wrapper for the Binect REST API (https://app.binect.de/index.jsp?id=api) that enables sending PDF documents as physical mail. The project consists of two artifacts:
 
-1. **Binect-JS SDK** (`@binect/js`) - A thin API wrapper
-2. **Binect Explorer** (`@binect/explorer`) - A browser-based interactive tool for learning and experimentation
+1. **Binect-JS SDK** (`@binect/js`) - A thin API wrapper in `src/`
+2. **Binect Explorer** - A browser-based interactive tool in `explorer/`
 
-## Architecture Principles
+## Architecture
 
 The SDK is organized around domain-aligned sub-clients that mirror the API vocabulary:
-- `documents` - PDF upload, status inspection, parameter modification
-- `attachments` - Attachment handling
-- `sendings` - Mail dispatch triggers, cancellation
-- `accounts` - Account management
-- `invoices` - Invoice access
+- `client.documents` - PDF upload, status inspection, parameter modification
+- `client.attachments` - Attachment handling
+- `client.sendings` - Mail dispatch triggers, cancellation
+- `client.accounts` - Account management
+- `client.invoices` - Invoice access
 
 ### SDK Layer Separation
 
-**Core API Layer** (authoritative): 1:1 semantic mapping to REST endpoints. Methods like `documents.uploadPdf`, `documents.getStatus`, `sendings.announce`, `sendings.cancel`.
+**Core API Layer** (authoritative): 1:1 semantic mapping to REST endpoints. Methods like `documents.upload`, `documents.get`, `sendings.send`, `sendings.cancel`.
 
-**Convenience Layer** (optional, non-authoritative): Additive helpers like status predicates (`isShippable`), error extraction, polling helpers. These must never be the only way to perform an action.
+**Convenience Layer** (optional, non-authoritative): Additive helpers in `src/helpers.ts` like status predicates (`isShippable`), error extraction, polling helpers. These must never be the only way to perform an action.
 
 ## Design Constraints
 
-- **No backend dependency**: Must function entirely in browser/JS runtime
+- **No runtime dependencies**: Uses native `fetch` API only
 - **No semantic reinterpretation**: Wrapper must not alter business meaning or outcomes
 - **Transparency over abstraction**: Developers must reason about actual API calls
 - **No default retries**: Network behavior must be explicit and opt-in
 - **Authentication**: HTTP Basic Auth, credentials are ephemeral (not stored/cached)
 
+## Key Files
+
+- `src/client.ts` - Main BinectClient class
+- `src/clients/*.ts` - Domain sub-clients
+- `src/types.ts` - TypeScript type definitions
+- `src/errors.ts` - BinectApiError and BinectAuthError
+- `src/http.ts` - Low-level HTTP client
+- `src/helpers.ts` - Convenience helpers (predicates, polling)
+- `explorer/index.html` - Standalone Explorer UI
+
 ## API Integration Notes
 
-- Uploads use base64-encoded PDFs
-- All non-success responses surface as structured errors preserving HTTP status, endpoint, and parsed response
-- The wrapper does not reinterpret business errors from the API
+- Uploads use base64-encoded PDFs (max 12 MB)
+- All non-success responses surface as structured `BinectApiError`
+- Document status codes: 1=Preparing, 2=Shippable, 3=Queue, 4=Printing, 5=Sent, 6=Canceled, 7=Error