PROMPT 05 Backoff And Closing

# Implementation Prompt 05: Backoff and Closing States

**Goal:** Implement :backoff state with exponential backoff and reconnection, and :closing state for graceful shutdown.

**Test Strategy:** TDD with rgr. All tests green, no warnings.

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## Context: What You're Building

You're implementing the final two states of the Connection state machine:
- **:backoff**: Exponential backoff with scheduled reconnection after failures
- **:closing**: Terminal shutdown state that ignores all operations except final cleanup

These states handle connection resilience and graceful termination.

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## Required Reading: State Transitions

From STATE_TRANSITIONS.md, these are the exact transitions to implement:

### :backoff State

| From | Event | Guard | Actions | To |
|------|-------|-------|---------|-----|
| :backoff | `{:state_timeout, :reconnect}` | - | Spawn new transport; wait for transport_up | :initializing |
| :backoff | `{:call, from}, {:request, method, params, opts}` | - | Reply with backoff error | :backoff |
| :backoff | `{:call, from}, :stop` | - | Reply `:ok`; exit(normal) | - |
| :backoff | `{:state_timeout, :tombstone_sweep}` | - | Sweep expired tombstones; reschedule sweep | :backoff |

### :closing State

| From | Event | Guard | Actions | To |
|------|-------|-------|---------|-----|
| :closing | `{:info, {:retry_send, _}}` | - | Ignore (retries cleared) | :closing |
| :closing | `{:state_timeout, {:request_timeout, _}}` | - | Ignore (requests cleared) | :closing |
| :closing | `{:state_timeout, :tombstone_sweep}` | - | Ignore (shutting down) | :closing |
| :closing | `{:call, from}, _` | - | Reply with shutdown error | :closing |
| :closing | `{:info, _}` | - | Ignore | :closing |

**Note**: :closing is reached via `{:stop_and_reply, :normal, ...}` so most clauses are defensive.

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## Key Requirements from ADRs

### ADR-0002: Backoff Delay Management

**Exponential increase on failure:**