E2E test: Subscribe with OffsetSpec::timestamp() — time-based offset

[s2x]

Closes #155

Problem

OffsetSpec supports TYPE_TIMESTAMP (0x0005) for time-based subscriptions, but no E2E test exercises this path. Time-based offset is important for replaying messages from a specific point in time.

Expected test

public function testSubscribeFromTimestamp(): void
{
    // 1. Publish messages
    // 2. Record current timestamp
    // 3. Publish more messages
    // 4. Subscribe with OffsetSpec::timestamp($recordedTimestamp)
    // 5. Verify only messages after the timestamp are received
}

Why it matters

• Time-based replay is a key feature of RabbitMQ Streams
• The OffsetSpec::timestamp() factory method exists but is untested E2E
• Serialization of timestamp offset type differs from other types

Acceptance criteria

• Subscribe with a past timestamp → receive messages from that point
• Subscribe with a future timestamp → receive no messages (timeout)
• Verify timestamp is correctly serialized as milliseconds since epoch