Simple intra-process messaging.

• Header-only – just drop one header (pubsub.hpp) into your include path
• Intrusive zero-copy messages – payloads live in place; hand-off is a single pointer move
• Bounded lock-free fast path – Boost.Lockfree ring for predictable tail-latency
• Overflow buffer – tiny mutex-protected queue absorbs rare bursts without dropping data
• Hierarchical topics – trades.BTC.fills style routing with fan-out to N subscribers
• CRTP convenience wrapper – derive, override on_message(), you’re done
• Optional Quill integration for structured, zero-alloc logging