test(01-01): add failing WriteBuffer unit tests

- Test offer/offerBatch/drain/isFull/size/capacity/remainingCapacity
- Tests fail because WriteBuffer class does not exist yet (TDD RED)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cameleer3-server-core/src/test/java/com/cameleer3/server/core/ingestion/WriteBufferTest.java

@@ -0,0 +1,99 @@
+package com.cameleer3.server.core.ingestion;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class WriteBufferTest {
+
+    private WriteBuffer<String> buffer;
+
+    @BeforeEach
+    void setUp() {
+        buffer = new WriteBuffer<>(10);
+    }
+
+    @Test
+    void offerSucceedsUntilCapacity() {
+        for (int i = 0; i < 10; i++) {
+            assertTrue(buffer.offer("item-" + i), "offer should succeed for item " + i);
+        }
+        assertEquals(10, buffer.size());
+    }
+
+    @Test
+    void offerReturnsFalseWhenFull() {
+        for (int i = 0; i < 10; i++) {
+            buffer.offer("item-" + i);
+        }
+        assertFalse(buffer.offer("overflow"), "offer should return false when buffer is full");
+    }
+
+    @Test
+    void offerBatchSucceedsWhenCapacitySufficient() {
+        List<String> batch = List.of("a", "b", "c");
+        assertTrue(buffer.offerBatch(batch));
+        assertEquals(3, buffer.size());
+    }
+
+    @Test
+    void offerBatchReturnsFalseWithoutPartialInsertWhenOverflow() {
+        for (int i = 0; i < 8; i++) {
+            buffer.offer("item-" + i);
+        }
+        // Buffer has 2 remaining, batch of 3 should fail entirely
+        List<String> batch = List.of("a", "b", "c");
+        assertFalse(buffer.offerBatch(batch));
+        assertEquals(8, buffer.size(), "no items should have been added on failed batch");
+    }
+
+    @Test
+    void drainReturnsItemsAndRemovesThem() {
+        buffer.offer("a");
+        buffer.offer("b");
+        buffer.offer("c");
+
+        List<String> drained = buffer.drain(2);
+        assertEquals(2, drained.size());
+        assertEquals(1, buffer.size());
+    }
+
+    @Test
+    void drainWithEmptyQueueReturnsEmptyList() {
+        List<String> drained = buffer.drain(5);
+        assertTrue(drained.isEmpty());
+    }
+
+    @Test
+    void isFullReturnsTrueAtCapacity() {
+        assertFalse(buffer.isFull());
+        for (int i = 0; i < 10; i++) {
+            buffer.offer("item-" + i);
+        }
+        assertTrue(buffer.isFull());
+    }
+
+    @Test
+    void sizeTracksCurrentDepth() {
+        assertEquals(0, buffer.size());
+        buffer.offer("a");
+        assertEquals(1, buffer.size());
+        buffer.drain(1);
+        assertEquals(0, buffer.size());
+    }
+
+    @Test
+    void capacityReturnsConfiguredCapacity() {
+        assertEquals(10, buffer.capacity());
+    }
+
+    @Test
+    void remainingCapacityDecreasesWithOffers() {
+        assertEquals(10, buffer.remainingCapacity());
+        buffer.offer("a");
+        assertEquals(9, buffer.remainingCapacity());
+    }
+}