fix: make SemanticVersionRange equality structural

The type was a record struct whose ==/Equals/GetHashCode were
synthesized from all fields, including the _sets array, so two
ranges that parse to the same constraint sets compared as
unequal (reference-sensitive on the array). This also broke
using ranges as dictionary keys, where lookup relies on value
equality.

Change the type to a plain readonly struct implementing
IEquatable<SemanticVersionRange> with structural equality over
the constraint sets (operation + version per comparator). The
custom ToString already existed, so no display behaviour is lost.

Add a failing-then-passing test.

CHANGELOG.md

@@ -17,6 +17,9 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Changed
 
 - `SemanticVersion` `==`/`Equals`/`GetHashCode` now ignore build metadata, matching precedence comparison rules.
+- `SemanticVersionRange` equality (`==`, `Equals`, `GetHashCode`) is now structural and based
+  on the constraint sets, so ranges that parse to the same comparators compare as equal
+  (previously it was reference-sensitive on the backing array).
 
 ### Fixed