Add "when is it public API for a trait to be implementable" blog post

It describes a number of edge cases around trait sealing that I discovered and had to reason through & implement in cargo-semver-checks. It includes the headline features of cargo-semver-checks v0.40.

Author: obi1kenobi

draft/2025-03-12-this-week-in-rust.md

@@ -38,6 +38,7 @@ and just ask the editors to select the category.
 ### Project/Tooling Updates
 
 ### Observations/Thoughts
+* [When is "this trait can be implemented" part of the trait's public API?](https://predr.ag/blog/when-is-trait-can-be-implemented-public-api/)
 
 ### Rust Walkthroughs