test(relaxng): cover include and externalRef resolution

Exercise RelaxNGParserIncludes (plain include, define override, externalRef)
through the schema loader; raises its line coverage from 24% to 81%.

Tests/RelaxNGIncludeTests.swift

@@ -0,0 +1,63 @@
+import Testing
+@testable import PureXML
+
+/// RELAX NG `include` and `externalRef` resolution (Sources/Schema/RelaxNGParserIncludes.swift),
+/// which a loader supplies. This path was under-covered; these exercise a plain
+/// include, an include with a `define` override, and an externalRef.
+@Suite("RELAX NG include / externalRef")
+struct RelaxNGIncludeTests {
+    private let namespace = "xmlns='http://relaxng.org/ns/structure/1.0'"
+
+    @Test("a plain include merges the loaded grammar's defines")
+    func test_plainInclude() throws {
+        let main = """
+        <grammar \(namespace)>
+          <include href='lib.rng'/>
+          <start><ref name='root'/></start>
+        </grammar>
+        """
+        let lib = """
+        <grammar \(namespace)>
+          <define name='root'><element name='root'><text/></element></define>
+        </grammar>
+        """
+        let schema = try PureXML.Schema.RelaxNG(main, schemaLoader: { $0 == "lib.rng" ? lib : nil })
+        #expect(try schema.validate("<root>hi</root>"))
+        #expect(try !schema.validate("<other/>"))
+    }
+
+    @Test("an include override replaces the included define")
+    func test_includeOverride() throws {
+        // The includer redefines `body`; the loaded grammar's `body` (which allows
+        // text) is dropped in favor of the override (which allows only empty content).
+        let main = """
+        <grammar \(namespace)>
+          <include href='lib.rng'>
+            <define name='body'><element name='root'><empty/></element></define>
+          </include>
+          <start><ref name='body'/></start>
+        </grammar>
+        """
+        let lib = """
+        <grammar \(namespace)>
+          <define name='body'><element name='root'><text/></element></define>
+        </grammar>
+        """
+        let schema = try PureXML.Schema.RelaxNG(main, schemaLoader: { $0 == "lib.rng" ? lib : nil })
+        #expect(try schema.validate("<root/>")) // override: empty content allowed
+        #expect(try !schema.validate("<root>text</root>")) // override removed the text pattern
+    }
+
+    @Test("an externalRef pulls a pattern from another document")
+    func test_externalRef() throws {
+        let main = """
+        <grammar \(namespace)>
+          <start><externalRef href='frag.rng'/></start>
+        </grammar>
+        """
+        let frag = "<element name='root' \(namespace)><text/></element>"
+        let schema = try PureXML.Schema.RelaxNG(main, schemaLoader: { $0 == "frag.rng" ? frag : nil })
+        #expect(try schema.validate("<root>x</root>"))
+        #expect(try !schema.validate("<nope/>"))
+    }
+}