Add minimal example: hello-spmc-ring-async

[lxsaah]

Difficulty: easy · Estimate: 2–3 hours

Async minimal example for the SpmcRing buffer (bounded stream with independent consumers — the most common buffer for sensor telemetry / event logs). The README quick-start already shows the pattern inline; this issue lifts it into a standalone runnable example so it sits alongside hello-mailbox and the future hello-single-latest examples as a complete minimal-per-buffer set.

Where to look

• README.md quick-start — the SPMC Ring example to mirror
• examples/hello-mailbox/ — minimal-example crate structure to mirror
• aimdb-core/src/buffer/cfg.rs — BufferCfg::SpmcRing

What to do

• Create examples/hello-spmc-ring-async/ with Cargo.toml, src/main.rs, README.md.
• ~30–60 lines of Rust: a sensor-style record (e.g. Temperature { celsius: f32 }) emitted on a timer by a .source(), with one .tap() printing each value as a typed bounded stream.
• Add to root Cargo.toml workspace members.
• Add to the Makefile's fmt and fmt-check package lists and the examples build target.
• README in the example explains SpmcRing semantics (bounded history, independent consumers) in one paragraph and mentions when to pick it over SingleLatest or Mailbox.

Done when

• cargo run -p hello-spmc-ring-async works and prints values
• make check and make all are green with the new crate wired in
• Linked from the main README's buffer table

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