fix: make ==/Equals ignore build metadata for precedence

Previously SemanticVersion was a record struct whose ==/Equals/
GetHashCode were synthesized from all fields, so versions that
differ only in build metadata were not equal. Range matching
already used precedence (ignoring metadata), creating an
inconsistency.

Change the type to a plain readonly struct implementing
IEquatable<SemanticVersion> and define ==/Equals/GetHashCode
in terms of SemanticVersionComparer.Priority, so build
metadata is ignored. The custom ToString already existed, so no
display behaviour is lost.

Add a failing-then-passing test.

CHANGELOG.md

@@ -16,6 +16,8 @@ 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.
+
 ### Fixed
 
 - Reject invalid SemVer identifiers when parsing versions.