ci: enable cargo-semver-checks now that 0.6.0 is published

.github/workflows/main.yml

@@ -57,15 +57,18 @@ jobs:
       # still packages, and it needs no credentials.
       run-publish-dry-run: true
 
-      # OFF until the first version is on crates.io. `cargo-semver-checks`
-      # diffs against the published baseline, and there is none yet -- turn
-      # this on once the first release-plz publish has happened.
+      # ON as of the 0.6.0 publish (2026-09-10), which gave
+      # `cargo-semver-checks` the crates.io baseline it diffs against. It was
+      # off before that because there was no published version to compare to.
       #
-      # Worth knowing when it is on: it reads the public API surface, so it
-      # catches a changed signature and not a changed behavior. A function
-      # that starts emitting different bytes passes it. Breaking changes of
-      # that kind still have to be marked `!` on the PR title by hand.
-      run-semver-checks: false
+      # Know what this does and does not buy: it reads the public API
+      # surface, so it catches a changed signature and not a changed
+      # behavior. A function that starts emitting different bytes passes it
+      # clean -- which is exactly what 0.6.0's `Message::encode` change was,
+      # so this check would not have caught the break that forced that bump.
+      # Behavioral breaks still have to be marked `!` on the PR title by
+      # hand; see CONTRIBUTING.md.
+      run-semver-checks: true
 
       # Both on as of the ported suites in `tests/property.rs` and `fuzz/`.
       #