[2/9] docs: add Backend enum tests and documentation

[unbalancedparentheses]

Summary

Add comprehensive tests and documentation for the Backend enum.

Tests (10 new tests)

• Backend enum traits (Default, Copy, Clone, Debug, PartialEq, Eq)
• All three backends handle call/cast correctly
• Backend::Thread isolates blocking work from async runtime
• Multiple backends can run concurrently with independent state
• Backend::default() works in start()

Documentation

• Comparison table: execution model, best use cases, limitations
• Code examples for each backend
• "When to Use" guide with advantages and when to avoid

~300 lines

Dependencies

⚠️ Merge after PR #105

Test plan

• All 26 tests pass

🤖 Generated with Claude Code

[ElFantasma]

I think this is not true: GenServer::start(Backend::Thread) uses GenServerHandle::new_on_thread which end's up creating a new Runtime (currently a tokio Runtime) in a dedicated OS thread. While this isolates it from other tasks in other threads, it still needs the async runtime. And I believe it currently needs some fine tuning and testing, as it is started via Runtime::new() which creates a multi_thread Runtime (probably unnecessary for this case) with all features enabled (which should be reviewed).

[ElFantasma]

This and the previous PR confuses between tasks::GenServerHandle::start_on_thread() and threads::GenServer::start():

tasks::GenServerHandle::start_on_thread(): Runs async code in its own Runtime in a dedicated thread

threads::GenServer::start(): Runs sync code in a dedicated thread without any async Runtime.

Both exist separately because even when their APIs are similar, the first one has some async functions while the later does not.

[ElFantasma]

Will approve and merge this one, it LGTM.