feat: add SSTable format and S3 persistence (Phase 1c)

Add persistence layer so MemTables flush to immutable SSTable files on
S3-compatible storage. When s3Bucket is not configured the store stays
in pure in-memory mode and all existing tests remain unaffected.

Key additions:
- ObjectStore interface with S3ObjectStore (MinIO) and
  FileSystemObjectStore (test double)
- SSTableWriter/SSTable: binary format with block index for range scans
- MergeIterator: K-way merge across MemTable + SSTables with
  deduplication and tombstone suppression
- Manifest: versioned JSON manifest tracking SSTables on S3
- S3ValueStore and S3NamespaceStore serialization/deserialization
- S3StoreConfig: S3 connectivity properties (bucket, endpoint, etc.)
- S3SailStore: flush path, merged read path, startup loading
- 29 new tests (unit + persistence); 541 total tests pass

Author: odysa

core/sail/s3/pom.xml

@@ -48,6 +48,10 @@
 			<groupId>com.google.guava</groupId>
 			<artifactId>guava</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>com.fasterxml.jackson.core</groupId>
+			<artifactId>jackson-databind</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>${project.groupId}</groupId>
 			<artifactId>rdf4j-sail-testsuite</artifactId>
@@ -71,5 +75,17 @@
 			<artifactId>junit-jupiter-params</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.testcontainers</groupId>
+			<artifactId>testcontainers</artifactId>
+			<version>${testcontainers.version}</version>
+			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>org.testcontainers</groupId>
+			<artifactId>testcontainers-junit-jupiter</artifactId>
+			<version>${testcontainers.version}</version>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 </project>

core/sail/s3/src/main/java/org/eclipse/rdf4j/sail/s3/S3NamespaceStore.java

@@ -10,11 +10,17 @@
  *******************************************************************************/
 package org.eclipse.rdf4j.sail.s3;
 
+import java.io.IOException;
+import java.io.UncheckedIOException;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map;
 
 import org.eclipse.rdf4j.model.impl.SimpleNamespace;
+import org.eclipse.rdf4j.sail.s3.storage.ObjectStore;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
 
 /**
  * In-memory store for namespace prefix information. All operations are synchronized for thread safety.
@@ -52,4 +58,37 @@ public synchronized Iterator<SimpleNamespace> iterator() {
 	public synchronized void clear() {
 		namespacesMap.clear();
 	}
+
+	@SuppressWarnings("unchecked")
+	synchronized void deserialize(ObjectStore objectStore, ObjectMapper mapper) {
+		byte[] data = objectStore.get("namespaces/current");
+		if (data == null) {
+			return;
+		}
+		try {
+			List<Map<String, String>> entries = mapper.readValue(data, List.class);
+			for (Map<String, String> entry : entries) {
+				String prefix = entry.get("prefix");
+				String name = entry.get("name");
+				namespacesMap.put(prefix, new SimpleNamespace(prefix, name));
+			}
+		} catch (IOException e) {
+			throw new UncheckedIOException("Failed to deserialize namespaces", e);
+		}
+	}
+
+	synchronized void serialize(ObjectStore objectStore, ObjectMapper mapper) {
+		try {
+			List<Map<String, String>> entries = new java.util.ArrayList<>();
+			for (SimpleNamespace ns : namespacesMap.values()) {
+				Map<String, String> entry = new LinkedHashMap<>();
+				entry.put("prefix", ns.getPrefix());
+				entry.put("name", ns.getName());
+				entries.add(entry);
+			}
+			objectStore.put("namespaces/current", mapper.writeValueAsBytes(entries));
+		} catch (IOException e) {
+			throw new UncheckedIOException("Failed to serialize namespaces", e);
+		}
+	}
 }