feat: initial approval workflow engine with in-memory and JDBC storage

Lightweight linear approval engine (v0.1):

- ordo-api: domain model, OrdoEngine port, repository SPI with
  conditional updates (insertIfAbsent, completeIfPending) and a
  TransactionExecutor port for atomic multi-step writes
- ordo-core: DefaultOrdoEngine running register/start/approve/reject
  inside a transaction boundary; in-memory engine and repositories
- ordo-storage-jdbc: thread-bound JDBC transactions, normalized V1
  schema migration, Jackson-based ProcessContext JSON codec
- tests: unit tests plus H2 integration tests; PostgreSQL integration
  tests run against a local instance via ordo.test.pg.* properties and
  skip when unreachable

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

ordo-example/pom.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.jetlumen</groupId>
+        <artifactId>ordo</artifactId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>ordo-example</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.jetlumen</groupId>
+            <artifactId>ordo-api</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.jetlumen</groupId>
+            <artifactId>ordo-core</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+    </dependencies>
+
+</project>

ordo-example/src/main/java/com/jetlumen/ordo/example/LeaveRequestExample.java

@@ -0,0 +1,32 @@
+package com.jetlumen.ordo.example;
+
+import com.jetlumen.ordo.api.ApprovalStep;
+import com.jetlumen.ordo.api.OrdoEngine;
+import com.jetlumen.ordo.api.ProcessContext;
+import com.jetlumen.ordo.api.ProcessDefinition;
+import com.jetlumen.ordo.core.InMemoryOrdoEngine;
+
+import java.util.List;
+import java.util.Map;
+
+public final class LeaveRequestExample {
+    private LeaveRequestExample() {
+    }
+
+    public static void main(String[] args) {
+        OrdoEngine ordo = new InMemoryOrdoEngine();
+        ordo.register(new ProcessDefinition("leave-request", "Leave request", List.of(
+                new ApprovalStep("manager", "Manager approval", "maria"),
+                new ApprovalStep("hr", "HR approval", "henry")
+        )));
+
+        var instance = ordo.start("leave-request", "alice", new ProcessContext(Map.of(
+                "requestId", "LEAVE-2026-001"
+        )));
+        var managerTask = ordo.findTasks(instance.id()).getFirst();
+        ordo.approve(managerTask.id(), "maria", "Approved by the manager.");
+        var hrTask = ordo.findTasks(instance.id()).get(1);
+        ordo.approve(hrTask.id(), "henry", "Leave record updated.");
+
+    }
+}