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>

research/binect-api-specification.md

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+# Binect API REST Specification
+
+**Source:** https://app.binect.de/binectapi/v1_swagger_api_kernel.json
+**Retrieved:** 2026-01-14
+
+## Overview
+The Binect API (v0.9.9) is a letter shipping service that handles document upload, validation, processing, and Deutsche Post delivery. The API uses HTTP Basic Authentication and HTTPS exclusively.
+
+## Base Configuration
+- **Base Path:** `/binectapi/v1`
+- **Host:** `app.binect.de`
+- **Scheme:** HTTPS only
+- **Authentication:** HTTP Basic Auth (required for all endpoints)
+- **Contact:** kontakt@binect.de
+
+## Core Endpoints
+
+### Documents Management
+**Upload & Retrieve:**
+- `POST /documents` – Upload letters/serial letters with base64-encoded content
+- `GET /documents` – List shippable documents (status 2)
+- `GET /documents/errors` – List erroneous documents (status 7)
+- `GET /documents/{documentID}` – Fetch specific document
+- `DELETE /documents/{documentID}` – Remove document
+
+**Document Attributes:**
+- `GET /documents/{documentID}/attributes` – Get all attributes
+- `POST /documents/{documentID}/attributes` – Set attributes
+- `GET /documents/{documentID}/attributes/{key}` – Get specific attribute
+- `PUT /documents/{documentID}/attributes/{key}` – Update specific attribute
+- `DELETE /documents/{documentID}/attributes/{key}` – Delete specific attribute
+
+**Document Modifications:**
+- `PUT /documents/{documentID}/transformations` – Apply scaling/offset adjustments
+- `DELETE /documents/{documentID}/transformations` – Revert to original
+- `PUT /documents/{documentID}/coverpage` – Add cover page
+- `DELETE /documents/{documentID}/coverpage` – Remove cover page
+- `GET /documents/{documentID}/pdf` – PDF preview
+- `GET /documents/{documentID}/png` – PNG preview
+
+**Attachments on Documents:**
+- `GET /documents/{documentID}/attachments` – Get document attachments
+- `POST /documents/{documentID}/attachments` – Add attachment to document
+- `PATCH /documents/{documentID}/attachments` – Update attachments
+- `DELETE /documents/{documentID}/attachments` – Remove attachments
+
+### Sendings (Shipments)
+- `POST /sendings` – Announce documents for delivery
+- `GET /sendings` – List all shipments (statuses 3-7)
+- `PUT /sendings` – Cancel unshipped letters
+- `POST /sendings/{documentID}` – Trigger single shipment
+- `GET /sendings/{documentID}` – Get specific sending
+- `PUT /sendings/{documentID}` – Cancel specific sending
+- `DELETE /sendings/{documentID}` – Delete sending
+- `POST /sendings/document` – Upload and immediately send
+- `GET /sendings/status` – Batch status check
+
+### Attachments (Standalone)
+- `POST /attachments` – Upload attachment
+- `GET /attachments` – List all attachments
+- `GET /attachments/{attachmentID}` – Get specific attachment
+- `DELETE /attachments/{attachmentID}` – Delete attachment
+- `GET /attachments/{attachmentID}/pdf` – Preview attachment PDF
+- `GET /attachments/{attachmentID}/documents` – Find associated documents
+- `PATCH /attachments/{attachmentID}/documents` – Attach to multiple documents
+
+### Account Management
+- `GET /accounts` – Retrieve credit/financial data
+- `GET /accounts/personaldata` – Get user details
+- `PATCH /accounts/personaldata` – Update user details
+- `GET /accounts/options` – Get default print options
+- `PUT /accounts/options` – Update default print options
+- `GET /accounts/coworkers` – List team members
+- `GET /accounts/coworkers/{debitornumber}/journal/{month}` – Coworker transactions
+- `GET /accounts/journal/{month}` – Account transactions
+
+### Invoicing
+- `GET /invoices` – List all invoices
+- `GET /invoices/{invoiceNumber}` – Fetch invoice transactions
+- `GET /invoices/{invoiceNumber}/pdf` – Download invoice PDF
+
+## Key Data Models
+
+### Document Status Codes
+- 1: In preparation
+- 2: Shippable
+- 3: Production queue
+- 4: Printing
+- 5: Sent
+- 6: Canceled
+- 7: Erroneous
+
+### Envelope Options
+- DINLANG
+- C4
+
+### Franking Types
+- UNSPECIFIED
+- STANDARD_FRANKING
+- DV_FRANKING
+
+### Production Countries
+- UNSPECIFIED
+- DE
+- AT
+
+### Response Format Options
+- FULL (default): Complete validation results
+- SHORT: Minimal response; validation runs asynchronously
+
+## Constraints
+- Maximum file size: 12 MB
+- Pagination supported via `limit` and `offset` parameters
+- Serial letters support token-based or page-count splitting
+- Transformations and cover pages can only be applied to original documents