Add GenericPhantom 3MF import mode with CT/DICOM export workflow

[jhajduga]

Add a new GenericPhantom mode for importing arbitrary 3MF/DB-based phantom geometries without constructing a patient object.
This enables running simple geometry-based simulations and generating synthetic CT/DICOM output directly from imported 3MF geometry.

Planned changes:

• add GenericPhantom as an IPhysicalVolume-based geometry module,
• allow PatientGeometry to run with m_patient == nullptr,
• add GenericPhantom_3mf envelope/build mode,
• make DB/3MF placement configurable via TOML,
• make scoring merge safe when no patient object exists,
• improve CSV and DICOM CT export logging,
• make Python DICOM export robust against unknown material names,
• extend HU dictionary with MaterialsSvc materials and basic biological tissue aliases,
• prepare a full README.md for new users explaining how to run a 3MF-based CT generation simulation.

The README should include a complete run instruction: required input files, example TOML config, build/run command, expected output directories, CSV CT export, and DICOM CT generation.

Future TODO:

• add sensitive-volume selection based on a configurable list of 3MF/DB element names or keys.

[jhajduga]

TODO:
Sensitive detector.
Upgrade build geo loop to allow reading from toml.