add spmc ring sync example

[harunugurlu]

Description

Adds the hello-spmc-ring example demonstrating the SpmcRing buffer through AimDB's synchronous API.

The example:

• registers a Temperature record backed by BufferCfg::SpmcRing { capacity: 10 }
• publishes sensor readings using a synchronous SyncProducer
• observes readings through configured fast and slow taps
• demonstrates how each subscriber maintains an independent read position in the shared bounded sequence
• shows the slow tap falling behind and later receiving every retained intermediate value in order
• documents the contrast with hello-single-latest, where intermediate updates may be replaced
• adds the example to the workspace, Makefile, and main README

Related Issue

Closes #184

Testing

The following commands completed successfully:

• cargo fmt --all -- --check
• cargo check -p hello-single-latest
• cargo clippy -p hello-single-latest --all-targets -- -D warnings
• cargo run -p hello-single-latest
• make examples

make check completed all preceding checks but failed at the existing codegen-drift step on macOS because of a BSD sed regular-expression compatibility error:

sed: RE error: invalid repetition count(s)
make: *** [codegen-drift] Error 1

Checklist

• I have read the [CONTRIBUTING.md](https://github.com/aimdb-dev/aimdb/blob/main/CONTRIBUTING.md) document.
• My code follows the project's coding standards.
• I have added tests to cover my changes. Not applicable: this PR adds a runnable documentation example rather than new library behavior.
• All new and existing tests passed (make check). The command is currently blocked by the macOS sed compatibility issue described above.
• I have updated the documentation accordingly.

[harunugurlu]

@lxsaah Hey, thanks for the review. I have fixed the duplication in Makefile and resolved the conversation. Now it still says "1 review requesting changes by reviewers with write access. Cannot change this locked branch" Thus, can't merge it.

[lxsaah]

I updated the branch and fixed the README output. It looks now good to me. Thank you!