E2E test: Subscribe with OffsetSpec::offset(N) — resume from specific offset

[s2x]

Closes #153

Problem

SubscribeTest covers OffsetSpec::first(), last(), and next(), but never tests OffsetSpec::offset(N) with a concrete offset value. This is the most common production scenario — consumers resuming from a stored offset after restart.

Expected test

public function testSubscribeFromSpecificOffset(): void
{
    // 1. Publish 10 messages (offsets 0-9)
    // 2. Subscribe with OffsetSpec::offset(5)
    // 3. Verify only messages from offset 5+ are received
    // 4. Verify the first received message has offset >= 5
}

Why it matters

• This is the primary production use case for stream consumers
• OffsetSpec::offset() serializes differently (TYPE_OFFSET = 0x0004 with uint64 value)
• Without this test, offset-based resume could silently break

Acceptance criteria

• Publish N messages, subscribe from offset M, verify only messages from M+ arrive
• Test with offset 0 (should behave like first())
• Test with offset beyond stream end (should receive no messages until new ones arrive)