test(xslt): cover result-tree namespace fixup generation paths (#319)

Adds XSLTNamespaceFixupTests for the previously-untested live paths in
XSLTNamespaceFixup.apply: generated ns0/ns1 prefixes for namespaced
attributes with no carried prefix, and xmlns="" undeclaration of an
inherited default namespace on an unqualified element (XSLT 1.0 7.1).
Exact serialized-output assertions, matching the existing XSLT test idiom.

Tests/XSLTNamespaceFixupTests.swift

@@ -0,0 +1,41 @@
+import Testing
+@testable import PureXML
+
+/// Result-tree namespace fixup (XSLT 1.0 section 7.1). A namespaced attribute
+/// with no usable in-scope prefix gets a generated `ns<n>` prefix and a matching
+/// `xmlns:ns<n>` declaration; an unqualified element reached under an inherited
+/// default namespace undeclares that default with `xmlns=""`. These run on every
+/// transform result (`XSLTNamespaceFixup.apply`, called from the serializer) but
+/// were previously only exercised on the prefix-already-present path.
+@Suite("XSLT result-tree namespace fixup")
+struct XSLTNamespaceFixupTests {
+    private let xsl = "xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\""
+
+    private func transform(_ template: String) throws -> String {
+        let style = """
+        <xsl:stylesheet version="1.0" \(xsl)>
+          <xsl:output method="xml" omit-xml-declaration="yes"/>
+          <xsl:template match="/">\(template)</xsl:template>
+        </xsl:stylesheet>
+        """
+        return try PureXML.XSLT.transform(stylesheet: style, source: "<x/>")
+    }
+
+    @Test("a namespaced attribute with no carried prefix gets a generated ns0 prefix")
+    func test_attributePrefixGenerated() throws {
+        #expect(try transform(#"<out><xsl:attribute name="foo" namespace="urn:x">v</xsl:attribute></out>"#)
+            == #"<out ns0:foo="v" xmlns:ns0="urn:x"/>"#)
+    }
+
+    @Test("two attributes in distinct namespaces take ns0 then ns1")
+    func test_twoAttributePrefixesIncrement() throws {
+        #expect(try transform(#"<out><xsl:attribute name="a" namespace="urn:one">1</xsl:attribute><xsl:attribute name="b" namespace="urn:two">2</xsl:attribute></out>"#)
+            == #"<out ns0:a="1" ns1:b="2" xmlns:ns0="urn:one" xmlns:ns1="urn:two"/>"#)
+    }
+
+    @Test("an unqualified element under an inherited default namespace undeclares it")
+    func test_undeclareInheritedDefault() throws {
+        #expect(try transform(#"<out xmlns="urn:d"><xsl:element name="plain" namespace=""/></out>"#)
+            == #"<out xmlns="urn:d"><plain xmlns=""/></out>"#)
+    }
+}