Expand book: Edition 2024, async, concurrency, exercises

[Gytisw]

Summary

• book.toml: Rust edition 2021 -> 2024; delete stale try!/edition2015 block and 2015-era extern crate comment.
• New std leaves: Cell/RefCell, OnceLock/LazyLock, Cow, more collections, iterators in depth.
• Concurrency: scoped threads + Send/Sync on Threads page; new shared-state page (Mutex, RwLock, atomics).
• New Async chapter (futures/await, spawning + channels, message streams) on tokio 1.x; runtime snippets are ignore-marked with Cargo.toml headers.
• New leaves: async in traits, GATs, const generics, error reporting (anyhow/thiserror), proc macros, cargo workspaces/tooling, MaybeUninit, property testing, Ecosystem CLI+JSON walkthrough.
• Uniform ### Exercise + Hint/Solution format on all new pages; retrofitted onto 5 existing testcase/activity pages.
• CI: MDBOOK_VERSION 0.5.1 -> 0.5.4 (matches local, avoids render drift).

Verification (stable 1.98.0, mdbook 0.5.4)

• grep -rn edition2015 src -> zero hits.
• mdbook test exit 0 (all new editable blocks included).
• mdbook build success; intra-book links checked locally (official linkchecker unfetchable here: GitHub API 401).
• Dependency/ignore snippets (tokio, anyhow, thiserror, serde, proptest, clap) run green pasted into a scratch crate.
• grep -rln '### Exercise' src | wc -l -> 24, each with Hint + Solution.

Translation stats (English-only new content; no po/ commits, fallback renders English)

• ja: 2761 translated, 151 untranslated.
• zh: 2959 translated.
• es: 456 translated, 3 fuzzy, 2459 untranslated (note: msgfmt reports 17 pre-existing fatal newline errors in po/es.po on master; untouched by this PR).
• ko: 2946 translated.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @marioidival (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @marioidival