feat: introduce IdentifierList for prerelease and build-metadata segments

Add a value type wrapping the dot-separated prerelease and build-metadata
identifiers, replacing the previous nullable string properties on
SemanticVersion. Implements IReadOnlyList<string>, zero-alloc
ReadOnlySpan<char> enumeration, Ordinal equality with implicit string
conversions, a JSON converter, and ISpanParsable/IParsable parsing
validated via IdentifierListFormatInfo.

Closes #1

CHANGELOG.md

@@ -17,13 +17,19 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - `SemanticVersion` and `SemanticVersionRange` can now be used as `System.Text.Json`
   dictionary keys. The custom converters override `ReadAsPropertyName`/`WriteAsPropertyName`
   so `Dictionary<TKey, TValue>` round-trips correctly.
+- New `IdentifierList` value type for prerelease/build-metadata identifiers.
 
 ### Changed
 
+- **Breaking:** `SemanticVersion.Prerelease` and `SemanticVersion.Metadata` are now `IdentifierList`
+  instead of `string?`. Implicit `string` conversions keep most call sites compiling, but the property
+  type change is breaking and requires a recompile. Constructors additionally accept `IdentifierList`.
 - `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).
+- `SemanticVersionComparer` now enumerates the `IdentifierList` segments directly instead of
+  re-splitting the joined identifier strings.
 
 ### Fixed