Add minimal example: hello-mailbox-async

[lxsaah]

Difficulty: easy · Estimate: 2–3 hours

The recently-merged hello-mailbox example demonstrates Mailbox buffer semantics using the synchronous API (AimDbBuilderSyncExt, producer.set(), consumer.try_get()). AimDB also has an async builder API (.source() / .tap()) — this is the API showcased in the README quick-start. Add an async sibling so users can compare both styles for the same buffer behavior.

Where to look

• examples/hello-mailbox/ — sync sibling; mirror its structure
• README.md quick-start — async builder pattern reference (.source() + .tap())
• aimdb-core/src/buffer/cfg.rs — BufferCfg::Mailbox

What to do

• Create examples/hello-mailbox-async/ mirroring the sync example's structure (Cargo.toml, src/main.rs, README.md).
• Implement the same "fire three rapid LED commands, observe only the latest" demo using the async builder:

  builder.configure::<Led>("actuator.led", |reg| {
      reg.buffer(BufferCfg::Mailbox)
          .source(|ctx, producer| async move { /* fire commands */ })
          .tap(|ctx, consumer| async move { /* observe latest */ })
          .finish();
  });
  builder.build()?.run().await?;

• Add to root Cargo.toml workspace members.
• Add to the Makefile's fmt and fmt-check package lists and the examples build target (see how hello-mailbox was wired in).
• README in the example explains how the async API differs from the sync sibling in 1–2 sentences.

Done when

• cargo run -p hello-mailbox-async works and demonstrates Mailbox latest-wins semantics
• make check and make all are green with the new crate wired in
• Linked from the main README's buffer table next to the sync version

We'd love help with this — comment if you'd like to take it.