Add PyGhidra snapshot loader

[lacraig2]

Summary

• add lazy dwarffi.ghidra program_to_isf converter for in-process PyGhidra snapshots
• add DFFI.from_ghidra(...) public API with currentProgram discovery
• cover converter behavior with fake Ghidra-like unit tests and a gated PyGhidra integration test
• document the PyGhidra snapshot workflow

Verification

• python3 -m ruff check src/dwarffi/ghidra.py src/dwarffi/dffi.py tests/test_pyghidra_loader.py
• python3 -m compileall -q src/dwarffi/ghidra.py src/dwarffi/dffi.py tests/test_pyghidra_loader.py
• PYTHONPATH=src pytest tests/test_pyghidra_loader.py tests/test_ghidra2isf.py

Full PYTHONPATH=src pytest was also attempted; existing dwarf2json-dependent tests fail in this checkout because dwarf2json is not available on PATH.