feat(pool): Add SMTPConnectionPool for high-throughput sending

[usernane]

Problem Statement

For applications sending many emails (notifications, newsletters, transactional), each message currently requires a full TCP + TLS + AUTH cycle (~200-500ms overhead). There is no way to maintain a pool of ready connections.

Proposed Solution

Add an SMTPConnectionPool class:

$pool = new SMTPConnectionPool($smtpAccount, [
    'max_connections' => 5,
    'max_messages_per_connection' => 100,  // Gmail limit
    'idle_timeout' => 300,  // seconds before closing idle connections
]);

// Usage
foreach ($emails as $email) {
    $email->send($pool->checkout());
    $pool->checkin();
}

$pool->closeAll();

Pool responsibilities:

• Maintain N pre-authenticated SMTPServer instances
• Check out idle connections for sending
• Send RSET and return connections after use
• Replace dead/stale connections transparently
• Track per-connection message count and cycle when limit reached
• Optionally send NOOP keep-alive on idle connections
• Enforce rate limits (max messages per second/minute)

Alternatives Considered

• External queue systems (RabbitMQ, Redis) — complementary but don't solve the connection overhead
• Single shared connection — doesn't support concurrency or resilience

Breaking Change

No — new class, no changes to existing API.

Additional Context

Depends on:

• feat(smtp): Add SMTPServer::reset() method to enable connection reuse #53 — SMTPServer::reset()
• feat(email): Allow Email::send() to accept an external SMTPServer instance #54 — Email::send() accepting external server

Most SMTP providers enforce per-connection message limits:

• Gmail: ~100 messages per connection
• Microsoft 365: ~30 messages per connection
• Amazon SES: ~50 messages per connection

The pool should respect these via the max_messages_per_connection config.