Harden the content contracts and enforce the slug rule

.editorconfig

@@ -54,3 +54,9 @@ dotnet_diagnostic.CA1812.severity = none
 # layout and is harmless (Blazor routing is not namespace-based). Renaming would
 # break the folder convention for no benefit.
 dotnet_diagnostic.CA1724.severity = none
+
+# CA1308 wants ToUpperInvariant over ToLowerInvariant. That advice is for
+# normalizing before a comparison, where a few characters do not round-trip. Here
+# lowercasing is the goal itself (URL slugs, which must be lowercase), not a step
+# before a comparison -- comparisons use StringComparison.OrdinalIgnoreCase instead.
+dotnet_diagnostic.CA1308.severity = none