[FEATURE] Add JPMS module descriptors

[DemchaAV]

Goal

Evaluate and add proper JPMS support for the published GraphCompose modules.

GraphCompose 2.0 is already split into separate Maven artifacts, but currently does not provide full module-info.java descriptors.

Requirements

• define stable module names;
• export only the public API and backend SPI;
• keep internal engine packages encapsulated;
• preserve support for third-party backends;
• declare uses and provides for ServiceLoader;
• verify dependencies work correctly on the module path;
• check for split packages;
• add a module-path smoke test with a custom external backend.

Compatibility

Third-party backends must remain supported. A consumer should still be able to depend only on graph-compose-core and provide their own renderer implementation.

Breaking change

• No expected public API break
• Possibly, depending on module-path compatibility findings