Fix GitHub issue #129: Add StringifiableTest for interface coverage

Created comprehensive test suite for Stringifiable interface to verify
constants and contract. This completes test coverage - every production
Java file now has a corresponding test file.

**Tests added (12 total):**
- testLineSeparatorPropertyConstant() - Verifies property name constant
- testLineSeparatorConstant() - Verifies line separator matches system property
- testLineSeparatorIsNotEmpty() - Validates line separator has content
- testDefaultPrefixConstant() - Verifies default prefix is 4 spaces
- testDefaultPrefixIsFourSpaces() - Validates prefix contains only spaces
- testInterfaceHasExpectedMethods() - Verifies all 3 toStringBuilder overloads exist
- testAbstractStringifiableImplementsInterface() - Confirms implementation relationship
- testLineSeparatorFieldIsPublicStaticFinal() - Validates field modifiers
- testDefaultPrefixFieldIsPublicStaticFinal() - Validates field modifiers
- testLineSeparatorPropertyFieldIsPublicStaticFinal() - Validates field modifiers
- testConstantFieldsAreStrings() - Verifies field types
- testInterfaceMethodsReturnStringBuilder() - Verifies method return types

**Benefits:**
- Complete test coverage pattern (every source file has test file)
- Constants are validated and documented through tests
- Interface contract is verified
- Regression protection for constants
- Executable documentation of interface design

Total tests: 277 (was 265, +12)
All tests pass with 100% coverage maintained.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Flossy

src/test/java/org/flossware/jcommons/StringifiableTest.java

@@ -0,0 +1,122 @@
+package org.flossware.jcommons;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Test class for Stringifiable interface.
+ * Verifies constants and interface contract.
+ */
+class StringifiableTest {
+
+    @Test
+    void testLineSeparatorPropertyConstant() {
+        assertEquals("line.separator", Stringifiable.LINE_SEPARATOR_PROPERTY);
+    }
+
+    @Test
+    void testLineSeparatorConstant() {
+        assertNotNull(Stringifiable.LINE_SEPARATOR_STRING);
+        assertEquals(System.getProperty(Stringifiable.LINE_SEPARATOR_PROPERTY),
+                     Stringifiable.LINE_SEPARATOR_STRING);
+    }
+
+    @Test
+    void testLineSeparatorIsNotEmpty() {
+        assertNotNull(Stringifiable.LINE_SEPARATOR_STRING);
+        assertFalse(Stringifiable.LINE_SEPARATOR_STRING.isEmpty());
+        // On Unix: \n, On Windows: \r\n
+        assertTrue(Stringifiable.LINE_SEPARATOR_STRING.length() >= 1);
+    }
+
+    @Test
+    void testDefaultPrefixConstant() {
+        assertEquals("    ", Stringifiable.DEFAULT_PREFIX);
+        assertEquals(4, Stringifiable.DEFAULT_PREFIX.length());
+    }
+
+    @Test
+    void testDefaultPrefixIsFourSpaces() {
+        for (char c : Stringifiable.DEFAULT_PREFIX.toCharArray()) {
+            assertEquals(' ', c, "Default prefix should only contain spaces");
+        }
+    }
+
+    @Test
+    void testInterfaceHasExpectedMethods() throws NoSuchMethodException {
+        // Verify all three toStringBuilder overloads exist
+        Stringifiable.class.getMethod("toStringBuilder",
+            StringBuilder.class, String.class);
+        Stringifiable.class.getMethod("toStringBuilder",
+            StringBuilder.class);
+        Stringifiable.class.getMethod("toStringBuilder",
+            String.class);
+    }
+
+    @Test
+    void testAbstractStringifiableImplementsInterface() {
+        assertTrue(Stringifiable.class.isAssignableFrom(
+            AbstractStringifiable.class));
+    }
+
+    @Test
+    void testLineSeparatorFieldIsPublicStaticFinal() throws NoSuchFieldException {
+        var lineSepField = Stringifiable.class.getField("LINE_SEPARATOR_STRING");
+
+        assertTrue(java.lang.reflect.Modifier.isPublic(
+            lineSepField.getModifiers()));
+        assertTrue(java.lang.reflect.Modifier.isStatic(
+            lineSepField.getModifiers()));
+        assertTrue(java.lang.reflect.Modifier.isFinal(
+            lineSepField.getModifiers()));
+    }
+
+    @Test
+    void testDefaultPrefixFieldIsPublicStaticFinal() throws NoSuchFieldException {
+        var prefixField = Stringifiable.class.getField("DEFAULT_PREFIX");
+
+        assertTrue(java.lang.reflect.Modifier.isPublic(
+            prefixField.getModifiers()));
+        assertTrue(java.lang.reflect.Modifier.isStatic(
+            prefixField.getModifiers()));
+        assertTrue(java.lang.reflect.Modifier.isFinal(
+            prefixField.getModifiers()));
+    }
+
+    @Test
+    void testLineSeparatorPropertyFieldIsPublicStaticFinal() throws NoSuchFieldException {
+        var propertyField = Stringifiable.class.getField("LINE_SEPARATOR_PROPERTY");
+
+        assertTrue(java.lang.reflect.Modifier.isPublic(
+            propertyField.getModifiers()));
+        assertTrue(java.lang.reflect.Modifier.isStatic(
+            propertyField.getModifiers()));
+        assertTrue(java.lang.reflect.Modifier.isFinal(
+            propertyField.getModifiers()));
+    }
+
+    @Test
+    void testConstantFieldsAreStrings() throws NoSuchFieldException {
+        assertEquals(String.class,
+            Stringifiable.class.getField("LINE_SEPARATOR_STRING").getType());
+        assertEquals(String.class,
+            Stringifiable.class.getField("DEFAULT_PREFIX").getType());
+        assertEquals(String.class,
+            Stringifiable.class.getField("LINE_SEPARATOR_PROPERTY").getType());
+    }
+
+    @Test
+    void testInterfaceMethodsReturnStringBuilder() throws NoSuchMethodException {
+        var method1 = Stringifiable.class.getMethod("toStringBuilder",
+            StringBuilder.class, String.class);
+        var method2 = Stringifiable.class.getMethod("toStringBuilder",
+            StringBuilder.class);
+        var method3 = Stringifiable.class.getMethod("toStringBuilder",
+            String.class);
+
+        assertEquals(StringBuilder.class, method1.getReturnType());
+        assertEquals(StringBuilder.class, method2.getReturnType());
+        assertEquals(StringBuilder.class, method3.getReturnType());
+    }
+}