feat(export): introduce LogExporter interface

## Summary

Refactor `sendLogs.ts` into a pluggable `LogExporter` interface. This is the architectural keystone for OTLP support and custom export targets.

## Motivation

Currently `sendLogs.ts` tightly couples batch management, transport (XHR/fetch/sendBeacon), and auth header construction. This makes it impossible to add new output formats (OTLP, custom backends) without duplicating the batch logic.

## Proposed Interface

```typescript
interface LogExporter {
export(logs: Logging.Log[]): Promise;
flush(): Promise;
shutdown(): Promise;
}
```

## Implementation

- `src/exporters/types.ts` — LogExporter interface
- `src/exporters/http-json.ts` — Extract current sendLogs behavior (default)
- `src/exporters/websocket.ts` — Extract current WebSocket behavior
- `src/exporters/index.ts` — Factory: config → exporter
- `src/sendLogs.ts` — Simplified to batch manager + exporter.export()

## Config Changes

```typescript
exporterType?: 'http-json' | 'otlp' | 'websocket' | 'custom';
exporter?: LogExporter; // for custom exporters
```

## Backward Compatibility

Existing users who set `url: 'https://my-server/logs'` must work identically. `HttpJsonExporter` is the default. No config change needed.

## Depends On

- v2.5.0 release (foundation)
- #137 (toolchain modernization)

Start by reading src/sendLogs.ts and tracing its current batch, transport, and authentication responsibilities. Review the proposed files—src/exporters/types.ts, src/exporters/http-json.ts, src/exporters/websocket.ts, and src/exporters/index.ts—along with the configuration changes. Done means the default HTTP JSON and WebSocket behaviors remain compatible while the exporter interface and custom exporter path are available; note that v2.5.0 and #137 are dependencies.

Written by the indexing model from the issue text.