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.

src/semver.tests/SemanticVersionRangeTests.cs

@@ -282,4 +282,14 @@ internal sealed class SemanticVersionRangeTests
 		await Assert.That(success).IsFalse();
 		await Assert.That(charsWritten).IsEqualTo(0);
 	}
+
+	[Test]
+	public async Task I_can_treat_structurally_equal_ranges_as_equal()
+	{
+		var a = SemanticVersionRange.Parse("^1.2.3");
+		var b = SemanticVersionRange.Parse("[1.2.3,2.0.0)");
+
+		await Assert.That(a == b).IsTrue();
+		await Assert.That(a.GetHashCode()).IsEqualTo(b.GetHashCode());
+	}
 }