docs(readme): document concurrency model and generation publication invariants

[GreyPikachu]

Description

This pull request completes the placeholder ## Engineering notes section in README.md by documenting WEB HTOP's concurrency invariants, ownership boundaries, and publication semantics.

Motivation

The README previously had a placeholder to be continued... under the Engineering notes section. Documenting the threading model, immutable generation publication, and backpressure design gives contributors and operators immediate clarity on concurrency safety and runtime overhead.

Changes

• Replaced the placeholder in README.md with concise documentation on:
  1. Thread ownership separation (std::jthread sampler vs epoll reactor).
  2. Lock-free immutable generation publication (PublishedSnapshot).
  3. Bounded per-session queues and latest-wins backpressure.

Verification

• Markdown formatting and layout verified.