PROMPT 01 Connection Scaffold

# Implementation Prompt 01: Connection GenStatem Scaffold

**Goal:** Create the `McpClient.Connection` module skeleton with data structure, callback mode, and init function. No state logic yet.

**Test Strategy:** Basic compilation and initialization tests pass. All tests green, no warnings.

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## Context: What You're Building

You're implementing the core Connection module for an MCP (Model Context Protocol) client library in Elixir. This module uses OTP's `gen_statem` behavior to manage connection lifecycle through explicit states.

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## Complete Specification

### States (from STATE_TRANSITIONS.md)

The state machine has 5 states:

| State | Description |
|-------|-------------|
| `:starting` | Initial state; spawning/attaching transport |
| `:initializing` | MCP initialize handshake in progress; initialize request sent |
| `:ready` | Normal operation; can process requests |
| `:backoff` | Exponential backoff; reconnection scheduled; transport inactive |
| `:closing` | Graceful shutdown in progress; no new operations accepted |

### Data Structure (from ADR-0003 and STATE_TRANSITIONS.md)