Add InMemory DAO package to turbo-core; document InMemory vs Spring-backed DAO differences

[Copilot]

The 7 in-memory DAO implementations existed only as private inner classes inside TurboEngineStandaloneTest, making them unreusable. Users encountering InMemoryFlowDefinitionDAO also had no explanation of what it is or how it differs from the Spring/MyBatis-backed version.

Changes

• New turbo-core/.../dao/memory/ package — extracts all 7 InMemoryXxxDAO classes into proper production code; each class carries Javadoc with per-method SQL equivalents and a comparison table vs. the Spring counterpart; package-info.java provides a package-level overview

// Zero-dependency bootstrap — no Spring, no DB config
ProcessEngine engine = TurboEngineBuilder.create()
    .flowDefinitionDAO(new InMemoryFlowDefinitionDAO())
    .nodeInstanceDAO(new InMemoryNodeInstanceDAO())
    // ...
    .build();

• TurboEngineStandaloneTest — replaces ~260 lines of inline inner-class DAO stubs with imports from the new package

• docs/standalone-usage.md — adds a dedicated section answering "what is InMemoryDAO and how does it differ from the Spring version": 7-dimension comparison table, side-by-side insert/update/select code, and a decision guide (Spring / InMemory / custom JDBC)

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