Make the Basic Event Loop example show the flow of control

[yukik8]

When I read the Basic Event Loop example for the first time, I couldn't tell what it was demonstrating: the final output is identical to what blocking IO would produce, and the most important fact — that io_wait suspends the reader without blocking the program — is not visible anywhere in the output.

This PR tries to make the flow of control visible:

• [main] / [reader] / [writer] labels show which fiber each line comes from.
• Comments mark exactly where the reader suspends and where execution resumes.
• The expected output shows [main] lines appearing while the reader is waiting — an ordering that would be impossible with blocking IO.

I verified the example runs unchanged on Ruby 3.4.10 (macOS arm64) and produces exactly the output shown. (This also sidesteps the Hash#inspect format issue addressed in #207, since the example no longer prints a Hash.)

Open question: I'm genuinely unsure about the trade-off — this version is much longer than the original 12 lines. Would you prefer:

• (a) this fully annotated version,
• (b) a shorter version with fewer log lines, or
• (c) keeping the original code and only adding an expected-output block?

Happy to rework it in any direction.

Types of Changes

• Maintenance.

Contribution

• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

Thanks for your suggestion, I made some minor refinement to make the output more concise, but overall this is a big improvement. Thanks.