fix: avoid infinite recursion when formatting ranges as short npm

TryFormat for the "ns" format returned false for any set that can
not be expressed as a caret/tilde range. The public ToString
routes through DefaultInterpolatedStringHandler, whose buffer-full
fallback re-enters ToString, causing an infinite loop / stack overflow.

For "ns", fall back to the full comparator ("n") form when the
compact form cannot represent the set, so the call always succeeds
for a representable range. A genuine buffer overflow still returns
false as before.

Add failing-then-passing tests.

CHANGELOG.md

@@ -20,6 +20,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - Reject invalid SemVer identifiers when parsing versions.
 - Range exact and non-equal comparators ignore build metadata when matching versions.
+- Avoid infinite recursion when formatting ranges in the short npm (`ns`) form.
 
 ### Removed
 

src/semver.tests/SemanticVersionRangeTests.cs

@@ -257,6 +257,21 @@ internal sealed class SemanticVersionRangeTests
 		await Assert.That(range.Contains(SemanticVersion.Parse("1.2.4"))).IsFalse();
 	}
 
+	[Test]
+	[Arguments("[1.2.3,1.4.0)", ">=1.2.3 <1.4.0")]
+	[Arguments(">1.0.0", ">1.0.0")]
+	[Arguments("[1.2,1.3]", ">=1.2.0 <=1.3.0")]
+	[Arguments("(,1.4.0]", "<=1.4.0")]
+	public async Task I_can_format_ranges_as_short_npm_even_when_not_caret_tilde(string range, string expected)
+	{
+		var value = SemanticVersionRange.Parse(range);
+		var destination = new char[256];
+		var success = value.TryFormat(destination, out var charsWritten, "ns", null);
+
+		await Assert.That(success).IsTrue();
+		await Assert.That(new string(destination[..charsWritten])).IsEqualTo(expected);
+	}
+
 	[Test]
 	public async Task I_fail_formatting_when_the_tentative_short_form_overflows()
 	{

src/semver/SemanticVersionRange.Formatting.cs

@@ -85,12 +85,29 @@ public readonly partial record struct SemanticVersionRange : ISpanFormattable
 
 				if (format is "ns")
 				{
+					var writtenBefore = buf.Written;
+
+					// Try the compact caret/tilde form first. If the set can't be expressed
+					// that way (TryFormatSimpleNpm returns false without writing anything) fall
+					// back to the full comparator form so we never report failure for a
+					// representable range - otherwise the public ToString would recurse
+					// infinitely via the interpolated-string handler's buffer-overflow fallback.
 					if (!TryFormatSimpleNpm(ref buf, sets[i]))
+					{
+						if (buf.Written == writtenBefore)
+						{
+							if (!TryFormatNormalNpm(ref buf, sets[i]))
 							{
 								return false;
 							}
 						}
 						else
+						{
+							return false;
+						}
+					}
+				}
+				else
 				{
 					if (!TryFormatNormalNpm(ref buf, sets[i]))
 					{